"Sexy women would 'happen' to be topless in the 1970s movies for no better reason than that everyone agreed, including themselves, that their breasts were a genuine pleasure to regard — the most beautiful naturally occurring shapes in nature, I believe. Now we see breasts only in serious films, for expressing reasons. There's been such a comeback for the strategically positioned bed sheet, you'd think we were back in the 1950s."
A nude character gets out of bed and wraps up in a sheet pulled from the bed to avoid exposing nudity to a sex-partner and, coincidentally, to the viewer.
Somebody, somewhere, may have actually done this in real life. Once. Maybe. It's of course largely illogical, as they've already seen each other completely naked. It's equally illogical when a character is actually all alone—although a chilly bedroom might be a possible explanation. In which case, you might expect her to wrap not a sheet but the whole duvet about herself - or to be wearing a big thick passion-killing flanelette nightie, which would be more typical of a woman sleeping alone on a cold night.
Also, when a man and a woman are nude in bed together, the sheet falls to the man's waist, but covers the woman's bust. Somebody in TV land must be selling L-shaped sheets. This version of the trope is also known as the His 'n' Hers Bedsheet.
Another way to serve the same purpose is to leave the actress' bra on (which is probably just as rare in real life as dragging the sheet off with you). Sarah Jessica Parker was criticized during the run of Sex and the City for relying on these techniques to avoid topless scenes, which all three of her co-stars performed.
Note also that women are rarely on top; it's harder to shoot a woman on top without showing her breasts. Missionary position is TV-friendly. The Modesty Bedsheet covers both partners in this position, and allows for lascivious hip churning while never actually showing anything explicit.
However, in somewhat more liberal shows, the female superior position becomes more common, shot from behind with a convenient bed sheet hiding anything you can't show on television. It seems to be a visual metaphor to indicate that this is much hotter and more passionate lovemaking than the plain old missionary position (and to suggest that the director is being risqué) while not showing anything more erotic than a bare back. Although that can be pretty erotic...
Even in pay cable shows such as Entourage and Californication, which freely show topless women, sex scenes frequently feature sheets tightly wrapped around the waists of the two actors simulating coitus in a manner that would be difficult and time-consuming to accomplish in real life.
There is also "Magic Boxers/Knickers", where after a night of passion, a character gets out of bed with their underwear on.
Also occurs outside strictly sexual settings, when characters who really should be nude in the context keep their underwear on. Examples: a corpse on a slab in the morgue being nude except for the underpants, and bathing/showering with underwear or swimwear on, even when the character is alone.
While this is a censorship trope, it is also a Fanservice one- a woman wearing solely a bedsheet is, well, a woman wearing solely a bedsheet...
Whether you believe it or not, this trope is actually Older Than Feudalism: There are numerous Roman wall paintings depicting couples during sex - with the woman wearing a brasserie, because it would've been considered lascivious for her to show her breasts. To her own husband. During sex.
See also Scenery Censor, Modesty Towel, Modesty Rags, and Right Through His Pants. Compare Toplessness From The Back, another way of showing nudity without showing nudity.
Ouran High School Host Club has a scene◊ where Tamaki rises from his bed suddenly and the sheet seems to just follow him up, wrapping him much like a Greek god. This is partially a parody, as the scene takes place in Tamaki's imagination.
Also parodied in Puni Puni Poemi, with what looks like a sex scene entirely covered by bedsheets, with legs randomly sticking out at the sides. When the sheets are pulled back, both participants are shown to be fully clothed and holding mannequin legs on sticks.
The same technique is used in Fullmetal Alchemist: It was just Hawkeye getting her feet licked by Black Hayate, causing her to make the strangely sexual giggling.
009-1: Mylene Hoffman/Agent 009-1 is shown using Modesty Bedsheets (combined with Toplessness From The Back) on several occasions. In fact, she was wearing nothing but a bedsheet in her very first appearance in the very first episode.
Witch Hunter Robin sleeps in the nude, which is surprising considering she was raised in a convent.
Well, the anime adaptation of Berserktried to use the modesty bed sheet (cape in this case) during Guts and Casca's love scene for a few seconds at least...
Same can be said for the manga too, though it served even less of a purpose.
In Wild Rose, Kiri uses one when he wakes up after transforming without any clothes on and wants to leave the manor. He does it again when Bernt enters his room without warning the morning after he sleeps with Mikhail.
Subverted by Kaoru in ...Virgin Love, who sometimes covers his torso with a bedsheet but is buck naked from the waist down. It's not clear what the bedsheet is doing, but it's not protecting his modesty by any means.
The fact that Todd and Damon did this in Manhunter came in handy for them when Todd's father walked in on them.
In one issue of Preacher, Jesse and Tulip are in bed, with the Modesty Bedsheet partially avoided: it covers them both below the waist, and we get to see Tulip topless. At the same time, the comic shies away from showing actual genitalia.
In at least one issue of Superman, Lois Lane and her husband reunite passionately. Afterward, Lois makes use of a Modesty Cape.
The Modesty Cape is reused in an utterly (albeit intentionally) ridiculous scene in The Dark Knight Strikes Again where Superman and Wonder Woman copulate whilst flying around the world (and at one point through a volcano) somehow without removing their costumes, but Supes' cape still hides the dirty deed all the same.
Used in the "Across Diamond Seas" story arc for Xanadu. Alicia modestly covers herself with a bedsheet when she comes to in the presence of the kirin who "rescued" her (long story there). She later uses the sheet as a make-shift dress until the kirin offers her more suitable clothing. Oddly enough, Alicia didn't seem that concerned with modesty in the "Thief of Hearts" story arch.
In one issue of Runaways, Nico and Victor are found in bed together after what is pretty clearly implied to have been a sexual encounter. However, both are fully clothed.
Paradoxically used in Irredeemable. Cutter gets a Modesty Bedsheet after sleeping with the Plutonian... even though she's an alien with Barbie Doll Anatomy who's always been nude in previous scenes.
This was already enough of a Discredited Trope in 1960 to be parodied in Shoot The Piano Player. Charlie is lying in bed, his chest uncovered, waiting for a prostitute. She crawls into his bed topless...and he thrusts the sheet into an l-shape over her breasts, saying "this is how you do it in the movies."
In America's Sweethearts, whose two lead characters end up sleeping together, the next morning not only is there the bedsheet, but both characters are partly dressed.
Used in the Iron Man movie, where Tony Stark's one-night stand wakes up and—possibly out of confusion and not knowing just who's going to walk in—grabs a sheet when JARVIS wakes her up. It's partially justified in that her clothes had been taken to be cleaned, so she doesn't have a choice for a cover up in a room with such a huge window. After that, however, she wears one of Tony's shirts (and nothing else).
In the Stephen Chow vehicle Tricky Brains, his character at one point wears a tan body-suit to a business meeting in a restaurant on which is written the characters for "I am naked."
Lampshaded in Bridget Jones: The Edge of Reason, when Bridget is getting out of bed trying to cover herself with the bedsheet, and her bedmate points out that it's a silly thing to do because "I happen to have a very high regard for your wobbly bits." She promptly decides to drop the sheet.
Probably lampshaded in Amazon Women on the Moon when the buxom Corinne Wahl's breasts fill the screen for several minutes prior to moderately explicit sex scene... when the sketch abruptly becomes a caught-cheating drama, they are never again seen, hidden behind the obligatory bedsheet.
One subversion used in film ('Running Scared', 'The Big Easy'), is where the woman wears a bedsheet to feign modesty in front of a visitor, not her sex partner. She is covered in the front while facing the visitor, but the audience get some fanservice because her rear end is uncovered in the back. This might need its own trope. Hmm.
In Mississippi Masala, when it shows Meena and Demetrius in bed after they have sex, they are both covered up by the sheet. Then, when Meena's father's friends knock on their hotel door, Demetrius gets up and pulls some pants on, but you can tell he's not wearing underwear. Then they barge in and Meena gets up and covers herself with a sheet, and it is obvious that she is naked under the sheet.
In Cabin Fever, an impulsive affair is shown with a bedsheet covering the rudest bits. Despite the woman's vigorous gyrations, the bedsheet amazingly stays in the exact same position. Reportedly, the actress doing all the thrusting, Cerina Vincent, struck a deal with the director to limit her nudity in the film, and did the scene with bedsheets literally duct taped to her posterior, to prevent them falling off.
Imediately following this scene is another where the same woman is washing her face in a bathroom. Suddenly, she's wearing panties, yet still no bra. Doesn't make a whole lot of sense when you considder the only person around for miles is the guy who just saw all her lady parts while they were getting their freak on.
In the movie Holes, Stanley showers in his swimming trunks... probably justified by the fact that he thought they were all under constant surveillance.
Well, and he was outside with barely any division from him and the desert and there is at least ONE woman at camp.
Shower shorts are a plot point of The Devil's Playground (1976), where seminary students are required to keep their bodies covered at all times. Dormitory scenes show the boys showering in swim trunks and dressing under bathrobes. The movie also averts the trope by showing the most prudish brother changing at a public pool.
Bizarrely played straight in Titanic with Jack and Rose in the back of a car. Not only is it uncertain where they got the blanket, but it took place not ten minutes after the famous nudie painting scene. Allegedly, this was because of James Cameron's tiptoeing around the censors, to still allow the movie to be PG-13. Nudity while posing for a painting probably counted as "tasteful", while nudity during coitus probably did not.
Claire later employs it when the nurse walks in the room.
Shows up from time to time in the James Bond series:
Played so straight in GoldenEye it almost seems to be lampshading this trope—the sheet is *extremely* low on Bond's waist, but they're practically up to the Bond girl's chin.
Averted in The World Is Not Enough, where the post-coital Bond and Elektra are snuggling in bed. He's lying on his side, cuddled up next to her, while she's lying on her stomach (resulting in Toplessness From The Back), with the sheet pushed down to BOTH of their waists. She turns over at one point and we get a brief glimpse of her breast before it cuts to a head shot. In light of this, it's played bizarrely straight later on when Renard and Elektra are also lying in bed together presumably post-sex, complete with her strutting around the room with a bedsheet specifically covering her no-no areas.
Inverted in Dagon, where the male character has the sheet most of the way up his chest, but the other half of the sheet is somewhere around the female character's hips or (depending on the point in the scene) knees.
Played straight in Goodbye Lenin. Alex and Lara fall on Lara's bed laughing. The next scene is the morning after and Alex gets out of bed still wearing his shorts, somewhat strange in a film that does feature full frontal male nudity.
Played ridiculously straight in The Ugly Truth. The two main characters have finally consummated their relationship. Two seconds after the Immodest Orgasm, Katherine Heigl's character is *still* wearing a bra.
Alan in The Hangover makes a sort of improvised bedsheet skirt when he can't find his pants.
Subverted in the Roger Corman films The Student Nurses and Candy Stripe Nurses. In The Student Nurses, the bedsheet's pulled up to Karen Carlson's waist when she goes to bed with the wrong doctor. When she gets busy with the right doctor she walks naked from the bed to the chair and turns on a lamp because she now insists on having sex with the lights on. In Candy Stripe Nurses, Candace Rialson's laying on a bed with the sheet pulled up to her stomach after having sex with a guy.
Utilized in the PBS filmed version of the musical Passion for the opening scene. While Clara and Georgio are shown having sex with her on top as the curtain rises, her back is to the audience, and the sheets are conveniently arranged so you never see Marin Mazzie's breasts. Averted in the actual play
Occurs in the Rocky Horror Picture Show after Frank catches Janet and Rocky having sex. But warranted, as the entire cast comes to gawk at them.
Laetitia Casta playing Brigitte Bardot does a phenomenally sexy dance with a modesty bedsheet in the French film Gainsbourg.
Averted (possibly accidentally) in The Woman in Red where Kelly Le Brock gets out of bed and pulls up the sheet to cover herself. Unfortunately - or perhaps not - the sheet briefly snags on something and while she is able to conceal her breasts, her pubic hair is visible.
Inverted in Scott Pilgrim VS The World. As Scott and Ramona lay in bed together, the sheet is pulled up over Scott's chest but down past Ramona's. She is wearing a bra, however.
Subverted by Rob Lowe and Kim Cattrall in Masquerade. They don't cover up after their romp in the sack.
In Deliver Us From Eva, Gabrielle Union & LL Cool J's characters Eva & Ray are so eager to get it on, they can't make it to the bed... but, despite all logic, find themselves, immediately after coitus, wrapped in sheets & blankets on the floor. They can't get IN the bed, but they can completely strip it? IIRC, Eva is also wearing panties.
Another Sarah Jessica Parker example: in the Bruce Willis vehicle Striking Distance, Parker and Willis begin to make out. Parker is wearing a backless dress and it's clear that she has no bra on. However, once she removes the dress, a bra magically appears.
Averted in Chaplin.
Ursula Andress in WW 1 air epic The Blue Max. There is a protracted scene where she has a folded towel looped around her neck so that the two lengths of towel, draped strategically in front of her, both conceal most of a breast, or at least the nipples. Despite how she moves or speaks, despite how her unsupported breasts jiggle and move, her nipples are always concealed by the towel as if it was glued in place (and probably was).
Used in the Adam Sandler hit Big Daddy when Sandler's Sonny is supervising his adopted son in the tub and makes him wear swim trunks because, "I don't know the rules on that." It's done because the completely innocent act of a father supervising a son's bath isn't so innocent on camera.
Near the end of To Green Angel Tower, when Miriamele and Simon finally have sex, and Josua comes in and wakes them up in the middle of the night, Miriamele keeps the bedsheet pulled up to her neck so she can hug him without compromising her modesty. Simon is left rather exposed by this, though.
Veronica Mars: "Mars, Bars". When we find Mac in bed with her boyfriend, the sheet covers him at the waist, though she has the sheet cinched tight clear up to her neck.
Buffy and Riley during much of "Where the Wild Things Are".
This happens all the time with Buffy and Spike in Season 6. One time they end up lying postcoitally on the floor in a wrecked building with an oriental rug covering Spike's naughty bits and covering Buffy almost up to the neck. Logistics of how they got under there aside, that's gotta be scratchy on the underside.
Genuine handmade ones are very soft and excellent for a round or two of coitus.
Justified though when Riley bursts in on Spuffy in "As You Were". Buffy is embarrassed so she cowers behind the sheets, while Smug!Spike taunts Riley by flaunting their relationship and his manly abs.
Subverted and lampshaded in Arrested Development by the character Tobias, who suffers from a personality disorder ("Never Nude") that requires him to wear a pair of tiny denim cut-off shorts at all times: in the shower, under his swimsuit, in bed, etc... (And yet, the fact that he has multiple pairs suggests he changes them somehow...) The irony is that his cutoffs are tight enough that he still frequently inadvertently exposes himself to his bunkmate George Michael (don't ask) and eventually gets his naughty parts broadcast on national TV. note His condition is that he can never be naked, not specifically that he has to wear cutoffs. Maybe he changes them while wearing a shirt or something.
In Cold Feet, Fay Ripley's chatacter is making love to her husband in the woman-on-top position. Fay said afterwards that the only reason it was shot from the back was not so much for greater erotic effect as to conceal the fact her not-inconsiderably-large breasts had to be seriously taped down, or they'd have been flying all over the place and quite possibly delivering a knockout blow to the poor chap. Or to Fay herself. This also precluded any chance of her going topless in front of camera.
The "showering with underpants" version is played with in an episode of Scrubs, where JD garners some odd looks for his Shower Shorts ("For the man who has nothing to hide- but still wants to!")
A series two episode of Torchwood features the "Magic Knickers" variation. Gwen has just had sex with her hubby Rhys and somehow managed to get her knickers back on before falling asleep (face down- she appears not be wearing anything else)
In the episode "Heart of Gold", Mal and the ex-Companion Nandi have sex in the female dominant position with a Modesty Bedsheet covering most of the action, but exposing Nandi's back.
Wash and Zoe also use one when they're in bed together (during "Shindig" and "Objects in Space")— in "Shindig" this sheet just happens to be laying over Zoe's body in such a way that it covers anything potentially inappropriate but leaves the length of her legs exposed. (Of course, considering they're Wash's favorite part of her body...*
Well, there and where her legs meet her back... actually, that whole area...
The opening to "Objects in Space" shows River sleeping naked, but she has a bedsheet covering her breasts. Summer Glau fans are most incensed.
A patient and his girlfriend start off one episode with a bang — while still mostly dressed.
House himself also engages in this, having sex with Lydia in the Season 6 episode Broken while both of them are fully clothed.
In police procedural shows like NCIS, the morgue bodies are often naked, but with very bright lamps directed on the naughty bits (thereby rendering them flat white), giving a sense of realism while appeasing the censors.
Starting with the sixth season, NCIS stopped using the lamps and simply covered the bodies with towels.
On female corpses, they are often shown with the chest cavity already open and a towel covering the lower half, thus using a sort of "Modesty Flesh" and reducing the need for a towel up to the neck.
Exceedingly bright lights are also used to wash out any naughty bits on either men or women cavaders that can't be reasonably hidden in the scene by a modesty sheet. Aparrently, the ceiling lights in the average autopsy lab kick out more light then the sun.
Also, in the Season 3 episode The Voyeur's Web, Tony and Ziva must fake having sex in order to pass for the married assassins they are impersonating. They alternate between the "completely covered by bedsheet" missionary position and Ziva on top, shot from behind. 5 seasons later, it's implied that they weren't faking after all.
In CSI: Crime Scene Investigation bodies being examined in the morgue are always covered with a modesty sheet, even during the forensic examination, which doesn't make sense. Sometimes they go to ridiculous extents to show injuries or bruises while keeping the sheet to avoid revealing too much of the body.
This is often averted with the original, opting instead to go with either selective blurring/framing to avoid showing any "naughty bits". Even still, at least on one occasion, a nipple of a female victim manages to get in-frame.
In the first episode of Secret Diary of a Call Girl, the main character appears to keep her g-string on during sex. Actual nudity doesn't appear until the third episode.
On The Riches Didi and her boyfriend use the Magic Knickers and bra/Boxers version despite just having had sex.
Lampshaded in a generic soap-opera parody sketch on Saturday Night Live, when anyone standing up in a post-coital bedroom scene would take the sheet with them without even having to use their hands (it was very obviously attached to the respective actor's undergarments in each scene).
Subverted in LOST with James and Juliette: when they wake up in the morning and he gets up to pick up the phone, he has a shirt on, and she, while having nothing top-region on, keeps lying with her back up (while half-asleep, so this even looks genuine). It's a shame that a mere couple of seconds later this trope is played straight when she gets up and pulls the sheets up along with her.
Played all too straight in the episode "The Economist", when Sayid is lying naked in bed with a woman whose navel is exposed, and yet has a sheet strategically placed across her bosoms.
The American version of Life On Mars had a particularly baffling instance of the magic bra. A Hooker with a Heart of Gold seduces the protagonist by taking her bra off in front of him (with her back to the camera) and jumping into his bed. In the following sex scene she's enthusiastically bouncing away on top of him... with her bra put back on to keep things US-TV-safe.
The "His n Hers Bedsheet" was used in FlashForward after Demetri and Zoey have sex. She covers her breasts, but Demetri is kind enough to let the sheet fall to his waist.
Star Trek: Voyager: "Dark Frontier". Whilst plotting the invasion of Earth, the Borg Queen brings up a holographic image of a human male wearing nothing but briefs. Given all the Body Horror normally associated with the Borg, this sudden show of modesty comes off as rather baffling.
Star Trek: The Next Generation: "Tapestry", Picard, while reliving his youth, has sex with his best friend. The next morning, Picard rolls over in bed to see Q laying next to him, on top of the covers. Startled, Picard pulls the sheet up (for some reason, over his chest) while talking to Q, until he realizes exactly what he has done, and relaxes hand a little.
Lois Lane uses one after having sex with Not-Clark in the Smallville episode Pandora.
In 24, Renee, having just had sex with Jack gets shot wearing one of these. It seems to cling on by sheer magic all the way to the hospital, turning what should be an emotional death scene into sheer narm.
The TV-friendly missionary position version is used in the interrupted sex scene in the Supernatural episode "The Monster at the End of This Book". This would work a lot better if one of the participants were not Lilith.
One particular episode of Friday The 13th: The Series featured an "artist" who made her statues by petrifying the models who posed for her. When this occurs on-screen, the model is wearing a bedsheet, which magically disappears once she becomes a nude statue.
The pilot of Grey's Anatomy started with this. Justified in that Meredith didn't really know who the guy was at that point.
In an episode of Covert Affairs, Auggie is on vacation and romances a woman. The next morning, she holds a bedsheet uncomfortably over herself while having a conversation with him. This is especially amusing because Auggie is blind, so she has no in character justification at all.
Invoked in Sherlock by the title character, of all people. In the middle of Buckingham Palace. Subverted in that far from having had sex, he was simply too lazy to put his clothes on before Mycroft's people hauled him off for a meeting. He also displays a perfect willingness to abandon the sheet as soon as it becomes an inconvenience.
Not exactly "perfect willingness"—he's quick to grab the sheet when it starts to slide off (because his brother stepped on it to keep him from walking away). He is, however, willing to use the threat of abandoning the sheet as leverage against said brother, who is already ticked off about his state of undress.
Parodied on an SNL sketch with Alec Baldwin showing various Soap Opera clips where both men and women walk away from the bed with a sheet already attached and covering their body without them holding it!
Justified in How I Met Your Mother both times Robin sleeps with The Casanova Barney when they aren't actually dating. She feels bad because she slept with her ex-boyfriend's friend and she usually mocks the type of chicks Barney sleeps with. The second time she was cheating on Kevin and Barney was with Nora.
Music and Music Videos
In the steamy video for her 1993 single "Soon", Tanya Tucker is wrapped in a bedsheet except for the scenes where she's shown in bed thrashing around with her lover.
Sheri in Pilgrims Musa and Sheri in the New World does this when Gamila walks in on her.
Happens a couple times in Xenogears, when Miang and Elhaym make sweet love to Ramsus and Fei respectively. Ramsus is revealed to be wearing Magic Briefs when he gets up for a walk, while Miang is happy to drop it and show her back to the camera a couple minutes later...and her front to Grahf on the other side. Elly, on the other hand, does the whole "sitting up in bed with the sheet pulled up over her breasts" schtick.
The Sims 2 has the Magic Briefs version; whenever a pair of Sims WooHoo, they switch clothes to their underwear while under the sheets.
Both varieties are used in How I Became Yours, both in chapter 4. L-shaped sheets are used in a scene between Zuko and Katara, and Toph covers her naked self with her bedsheets just before the Taang love scene.
In Out There, characters (mostly the females) are sometimes shown bathing, undressing, or lounging in bed. Usually they are seen from the shoulders up, or from the rear (so that only a naked back is visible); otherwise, a bedsheet provides cover. Interestingly, Rebecca and Wally, the only married couple in the strip, are also the only people ever portrayed under the sheets together; non-married couples are usually seen fully clothed, laying on top of the sheets 
Used in the last comic on this page of Skin Horse, though the bedsheet goes almost up to the man's chest. A more traditional female-chest/male-waist version appears in the following comic (though perhaps justified in that the woman is somewhat ashamed of what she did).