Rare Female Example: Sae in Hidamari Sketch constantly insists she has had tons of boyfriends and has all kinds of experience which she supposedly uses as inspiration for her writing.
The plot of Love Lab centers on this trope, as main character Riko lies about how she's basically an expert on love matters and is made to 'teach' others on how to be experts as well. Eventually, the trope is subverted when she admits to having zero experience in love, but that doesn't happen until the very end of the anime.
Similarly, Allison lies that she is a nymphomaniac who even slept with her therapist before admitting that she is actually a virgin after Claire does.
Rascal in Memphis Belle constantly teases "Virgil the Virgin" while bragging about his own conquests. The novelization reveals that Rascal himself is in fact a virgin (ironically Virgil is not, having slept with a woman the day before the mission).
In The Departed, The Mole's new marriage is suffering from The Loins Sleep Tonight. When his boss says that women are attracted to married men because "it tells them you've got some money and your dick works," he responds, "Oh, it's working! Ha-ha! Overtime!"
Carlos Ramirez from The Dresden Files. He talks a big game but when they show up at the Raith estate in the run up to the climax in White NightLara Raith is touched that Harry thought to bring a virgin as a gift.
Daniel Radcliffe guests starred in an episode Extras as himself - a pathetic version of himself who is desperate to shag anything that moves and to shake off his Harry Potter persona by doing such "adult" things as smoking.
Radcliffe: I've done it with a girl, intercourse-wise.
Parodied with Dave in Flight of the Conchords, who acts like an expert in all things, especially being a ladies' man. Judging by his tendency toward malapropisms and the way he (deniedly) lives with his parents...
Kirk St Moritz from Dear John was always bragging about his non-existent conquests.
In an episode of Quantum Leap the son of the person Sam leapt into was a virgin and his "friends" mocked him for it, but according to Word Of Ziggy one of them was a virgin too.
In The IT Crowd, Moss does this when he has to pretend to be married, several times going out of his way to mention all the frequent and amazing sex that goes on in his marriage.
Xander's classmate in the Buffy the Vampire Slayer episode "Teacher's Pet". We know this because he becomes the prey of a Monster of the Week that only eats male virgins. He threatens to sue everyone when they figure this out.
The Todd from Scrubs is known across Sacred Heart and beyond for his high fives and constant sexual innuendo. When finally asked by the Janitor how many times he had sex recently the answer was a shame filled "Bagel."
In the unaired pilot for the TV adaptation of Global Frequency, the main female lead is listing her very lengthy list of academic qualifications, all of which she has achieved at a very young age (she's no older than thirty). The male lead makes a joke along the lines of "how'd you find time to have a life?" The female lead, suddenly very touchy, begins insisting that she had a life, ending with a completely out of the blue and not very convincingly delivered declaration that she "had boyfriends," the implication being that she's lying and is probably still a virgin.
Mini McGuinness from the third generation of Skins.
Parodied in a SNL sketch where Paul Reiser As Himself is interviewed on a talk show and is asked how many sexual partners he's had in his life. He answers three dozen, which the host then mishears as three, and proceeds to deconstruct ruthlessly, explaining that all men automatically exaggerate their numbers, so "three might really mean just two, one, or even zero." A group of proud virgins in the audience stand up to congratulate him and immediately make him their rolemodel to his utter dismay and virulent protest.
In one episode of How I Met Your Mother, we get a flashback to Barney's school days. One of his classmates (probably around twelve years old) brags to Barney that he's slept with a hundred girls. Barney takes this at face value and retorts that someday, he'll sleep with two hundred girls. In a rare subversion of this trope, he actually does. When he calls up the old classmate to gloat, the classmate is quite disturbed and says that Barney needs help.
Sheldon and Amy from The Big Bang Theory do this to convince Amy's mother that they are a couple, as she is uninterested in romance or sex (at first).
In a later season, Sheldon is forced to work with a colleague he doesn't like. When he discovers that the other guy's work is more advanced than his, he is humiliated. The other guy says it's understandable, since Sheldon has a girlfriend to distract him. Sheldon goes along with the assumption, but unfortunately now the colleague wants to hear details...which Sheldon is far too clueless to provide. Hilarity ensues.
30 Rock: Liz says of Jack's career troubles that he's having a tough year: "First William F. Buckley dies, now this? Next stop impotence, right?" In response, he just laughs way too loudly and leaves the room.
"Kif, I have made it with a woman. Inform the men."
Although ironically, that quote comes from right after he really did have sex with a woman. And they arecertain it's a woman this time. Still, a later episode suggests this trope applies to him more often than not:
Zapp: You're the only woman who's ever loved me!
Leela: I never loved you!
Zapp: I mean physically!
Master Shake from Aqua Teen Hunger Force likes to portray himself as a ladies'... shake, although it is repeatedly lampshaded by someone (usually Frylock) pointing out that he doesn't even have genitalia or any knowledge of how sex even works.
In one episode of South Park, Cartman claims he's been laid "like, five thousand times."