Sometimes a character really does want to just be friends with another; at least as far as conscious decisions go, but the sexual attraction and sexual thoughts persist.
Maybe something about the friend, whether appearance or personality or whatever else, just seems sexy. Perhaps the character is so dirty-minded by their very nature that they just have sexual thoughts anyway. Whatever the case, the lust conflicts with the friendship, no matter how hard they try to conceal the former.
Compare and contrast with I Don't Want to Ruin Our Friendship. The difference lies in the reaction of the character feeling the romantic/sexual feelings (and, in many cases, the former relationship between the two): if they're creeped out by their sudden attraction to the person they had always considered a brother/sister, it's this trope; if they're worried that admitting their feelings to someone with whom they've been Just Friends will cause drama or change a group dynamic, it's I Don't Want to Ruin Our Friendship.
Compare Stupid Sexy Flanders (of which this is a snowclone of), as it reflects on a similar conflict between the sexual thoughts one has and consciously thinking one shouldn't have such sexual thoughts. Compare and contrast Friends with Benefits, one solution to this problem if it's mutual.
Truth in Television, of course. After all, it's considered normal to lust after someone you don't know, as opposed to a close friend (that you do know).
- In Persona 5: The Animation, the Phantom Thieves visit a bathhouse, with the Ren, Ryuji, and Yusuke on one side of a wall and the sole female Thief Ann on another. While the boys can only hear Ann humming through the wall, they can't help but picture what it's like on her side, complete with Imagine Spots of Ann going full Ms. Fanservice.
- In Yuri series Whispered Words, Sumika Murasame is stuck being Just Friends with Lipstick Lesbian Ushio Kazama, but as the story goes on, we find out that Ushio has fallen for Sumika at some point but is extremely uncomfortable being attracted to a close friend.
- In Infinite Stratos, this is virtually Ichika's relationship with all the members of his Unwanted Harem: he has shown some interest in all of them, ranging from cuteness (Rin, Laura, Char), to fascination of beauty (Cecilia, Houki on certain instances) all the way to outright sexual curiosity (Houki, probably Char). However, he has not decided to choose from among them due to his circumstances. Unfortunately, the girls themselves seem to misinterpret his actions as that of being a very dense oaf.
- This happens to both Ishigami and Fujiwara on separate occasions in Kaguya-sama: Love is War when they're thinking about the kind of person they'd want to have a relationship with (Fujiwara for Ishigami, and Shirogane for Fujiwara). Both immediately deny the possibility of such a relationship happening.
- Shinichi from Uzaki-chan Wants to Hang Out! will begrudgingly admit that Hana is his friend but he will absolutely deny that they're dating or that he's even attracted to her to the point where he will immediately look away when he finds himself gawking at her. As of chapter 69, he's had a Love Epiphany.
- Handsome Girl And Crossdressing Boy: Everyone thinks Iori is cute as a girl, and in the very first page one of the friends who made him do it in the first place is thinking "I'd tap that".
- While Carol and Harry are pretty firm about being Just Friends in Child of the Storm, there are persistent mild shippy undertones to their friendship, and it's implied that while Harry's fairly new to the whole sexual attraction thing (he's thirteen when the story starts), he wouldn't mind if they got a Relationship Upgrade. As it is, however, he seems equally happy with being Just Friends, since Carol has about two actual guy friends who aren't trying to get in her pants and very few friends full stop. In chapter 46 of the sequel, they end up getting together.
- A lesbian example of the trope occurs in Killswitch, the third book in the Cassandra Kresnov trilogy by Joel Shepherd. The titular android's bisexual best friend admits to having fallen in love with Cassandra, but the latter is straight and the friend doesn't want to affect their working and non-working relationships.
- Between Lila and her Captain, Alucard in A Darker Shade of Magic, judging by his nearly successful attempt to seduce her for information.
Lila: You sure know how to make a girl feel special.
Alucard: I thought I was in trouble precisely for making you feel special.
- At the beginning of Dora Wilk Series, Dora tries to keep things friendly between her and Miron, but both have rather sexual thoughts of one another. The problem here is mostly that Dora, as a succubus expy, is used to people lusting after her because of her powers and thus thinks that her relationship with Miron would be result of magic rather than actual attraction. They do get together in the third book.
- The Dresden Files:
- This is part of the relationship between Harry Dresden and Molly Carpenter. He doesn't reciprocate her feelings towards him (partly because he feels it would be creepy to do so, given that he's known Molly since she was a child, partly because her father is the big brother/cool uncle he never had), but still finds her physically attractive. She, on the other hand, is carrying a torch for him the rough size of an apartment building (though granted, a later short story implies that she's finally developed more of a Like Brother and Sister-style dynamic with him). Understandably, awkwardness ensues.
- Something similar exists between Harry and Karrin Murphy, as they have been close colleagues since the first book and gradually getting closer in other ways, while both are aware that the complications of a relationship would be dangerous for both of them. At the end of Skin Game, Harry and Murphy get together.
- The Kharkanas Trilogy: The cousin version. Faror Hend struggles with her lustful feelings towards her younger cousin Spinnock Durav, who in turn thinks he can act freely around her as she is his cousin and thus in no danger of misunderstanding his smiles and flirty remarks. He is suitably squicked when she confronts him.
- For a brief period Angel's feelings towards Cordelia are this, with him sort of horrified that he does feel something romantic for her when previously they were like family. By the time he gets over it there's other issues preventing them getting together... not that there weren't already.
- Buffy the Vampire Slayer:
- This is pretty much how Xander feels about Buffy after he gets past being outright attracted to her and too inept to say so.
- Willow finally gets together with Xander... when they're both with other people.
- In Community, Jeff and Annie often fall into this territory. They're pretty much the Official Couple by this point, but the age difference and nature of the relationship so far has made them both feel somewhat squicky about the idea of hooking up, despite how much they both seem to want to hook up.
- In Doctor Who, the Eleventh Doctor and Clara have problems with this towards each other, due to some minor Unresolved Sexual Tension between both of them. (They both insist on only being friends, as much as they like each other.) The trope is more prominent on the Doctor's side and usually Played for Laughs, while Clara off-handedly mentions once (while alone) that she has to keep herself in check and not develop a crush on him. Later, though she's not attracted to the older-looking Twelfth Doctor, she still makes a cheeky compliment about his handsome looks and charismatic voice, much to his annoyance.
- The Good Place: Eleanor has this kind of relationship with Jason. While she's repeatedly noted that he's quite easy on the eyes, she also makes sure to squash her lusty thoughts as soon as they pop up since they're otherwise Like Brother and Sister along with both of them being seriously involved with other people (Chidi & Eleanor and Jason & Tahani/Janet).
- All three of the male roommates on New Girl have had moments like this with their female roommate Jess. This is especially the case for Nick, who has some serious Unresolved Sexual Tension with her.
- On NCIS: Los Angeles: It's a quick moment, but it's there. When Kensi is compelled to do a quick change in clothes in the middle of an undercover op, Callen's eyes widen. Then he politely looks away.
- Schitt's Creek: This becomes a thing for David and Stevie after their Friends with Benefits arrangement ends. In early seasons, David takes notice of her when she dresses up, and they even try a kiss to rekindle the relationship, which turns out badly. Any residual tension pretty much ends when David meets Patrick, but the pair have a few wistful moments of what might have been in the episode where Stevie learns of David and Patrick's engagement.
- In Little Busters!, if you choose to stay with the guys rather than go with the girls on their sleepover, they end up conversing about love and when Riki is evasive, Masato jokes that he has a crush on Kyousuke. If you choose to lie, Riki admits mentally that he does, but denies it because he doesn't want to ruin their friendship. Then when he asks Kyousuke who he likes, Kyousuke replies 'right now, Riki is my number one', causing Riki to blush heavily. However, he berates himself for thinking that way, stating that indecent thoughts like that are desecrating their friendship. Immediately afterwards Kyousuke asks him "Wanna do it?" referring, of course, to playing tabletop baseball.
Riki: Uooooh, what a misunderstanding I just had!! It's a desecration of our friendship... God of friendship, forgive me...
- In Morenatsu, it's revealed that Shin feels this way about you, which makes his self-conscious issues even more frustrating for him.
- In All New Issues, Todd and Robyn look like they're almost going to kiss, and then this trope kicks in. In fact, they're the trope image.
- In Sunstone, Anne reading Lisa's bibliography of self-starring kinky erotica served as a good way to explore her interest in BDSM, but also, to Anne's annoyance, a good way to explore an interest in Lisa.
- In El Goonish Shive, Sarah still lusts after Elliot even though she's no longer interested in being Elliot's girlfriend anymore.
- Linda to Francine in American Dad! What makes this more tragic is Linda is unaware that Francine is bi and doesn't seem to consider lesbian affairs as cheating.
- Fish Police:
- Detective Gil is happily in a relationship with diner waitress Pearl and doesn't want to be anything but good friends with Angel. The problem is that Angel is a blonde bombshell who is widely regarded as the most beautiful woman in town, who unlike Gil 'does' want to be something more with him. Despite his insistence that they're just friends, Gil can't help but make frequent inner monologues about how beautiful Angel is.
- Pearl also counts Angel as a friend. Though Angel seems more invested in being Pearl's friend than the other way around. The problem is that Pearl is aware that her boyfriend is physically attracted to Angel (because most guys in the city are) and that Angel has feelings for Gil. She swings between jealousy towards Angel and anger at Gil.
- Star vs. the Forces of Evil: Despite canonically having crushes on other people, both Star and Marco each show an awkward physical attraction to the other at different points. Perhaps inevitably, Season 2 has Star develop a complicated crush on Marco that even she isn't really aware of at first. Then in Season 3, just as Star is trying to get over Marco, Marco realizes he's got a crush on Star. In Season 4, they finally step aside and talk to each other openly about their feelings, and officially become a couple just before the series finale.