The character is emotionally unable to love and have sex with the same person. This is often caused by a deep sexual self-loathing where they consider their own sexuality to be dirty and may be a dysfunctional way of getting over a Paralyzing Fear of Sexuality.
For a male character, it often comes with a mindset of My Girl Is Not a Slut: Either in the form of "I don't want to make my girl a slut by sleeping with her" or in the form of "she's a slut for sleeping with me, so I don't want her". This phenomenon was formerly popularly called the MadonnaWhore Complex, back before that other Madonna's frank sexuality made the old term a bit too confusing.
For a female character, the same problem may apply through Internalized Categorism. It may also be about being overly concerned with avoiding the Sex for Services trope: If a guy does nice things for her, she might be afraid of coming across as paying for his services with her body, and thus she'd rather have sex with some random jerk she's not even as attracted to.
Of course, for both genders, the portrayal may very well limit itself to the character being messed-up in general. Or simply portrayed as inherently incomprehensible.
Since most people are capable of being emotionally and/or sexually attached to more than one person, it's not an example when a character simply has sex with someone other than the Love Interest. For this trope to come into effect, the separation between lust and love has to become a problem. If not emotionally, so at least socially.
Compare/contrast Can't Have Sex, Ever and Sex Equals Love. It can often accompany Sex for Services. See also Can't Act Perverted Toward a Love Interest. For cases where a woman falls for a man other than the Protagonist-Centered Morality dictates that she ought to, see instead Dogged Nice Guy or All Girls Want Bad Boys. Chastity Couple, Courtly Love, Sex Changes Everything, and Friends with Benefits are subtropes.
- In the Swedish strip Rocky, one of the titular character's friends fall in love with some random woman and doesn't even want to have sex with her because he considers her "too pure" for that. The whole thing is Played for Laughs, with him portrayed as delusional and she's portrayed as a total skank.
- The My Little Pony: Friendship Is Magic fic Language involves Rarity falling in love with Fluttershy, even though she'd thought herself heterosexual up to this point. The (NSFW) sequel Lust confirms that Rarity is bi-romantic but still heterosexual — so she's emotionally in love with Fluttershy, but not sexually attracted at all. Since Fluttershy is very much attracted to Rarity in both ways, this causes problems, and the bulk of the story is Rarity and Flutters working through the conflict.
- Implied in Shame. Brandon appears to have difficulty performing with women he is emotionally intimate with.
- In Summer of Sam, Vinny is too locked into his Madonna/whore complex to have good sex with his wife, and cheats on her frequently.
- Michael's real father got himself into big trouble due to this in The Beast Within. He slept with a woman whose husband was such a prude that he refused to have sex with his own wife, and when he caught them in his bed, he killed his unfaithful wife, and chained Michael's father to his basement's wall to make him suffer a slow death.
- Florence Foster Jenkins: St. Clair has a happy, chaste marriage with Florence, and sees a mistress on the side. He's framed as a Sympathetic Adulterer (and Florence is implied to overlook the arrangement willingly) because Florence is seriously ill with long-term syphilis from her first husband, so sex isn't an option for them.
- In Shimoneta, Tanukichi Okuma was in love with Anna Nishikinomiya because of her pureness and tender personality, so he joins to the same high school than Anna to be closer to her. But after she became a nynphomaniac yandere that tried to rape him several times, Tanukichi started to feel the opposite about her and tried to avoid her like the plague.
- In the Dresden Files, Justine and the White Court vampire Thomas Raith truly love each other... which means The Power of Love burns any vampire who touches her, including him. He still needs to feed on Life Energy, though, so he has any number of casual partners.
- Victor, a sex addict, in the Chuck Palahniuk novel Choke (as well as The Film of the Book starring Sam Rockwell) can't sleep with his mother's doctor Paige, for this reason.
- In The Millennium Trilogy, Lizbeth Salander makes a very clear delineation between the people that she has sex with and those whom she actually cares about, and is alarmed when Mikael Blomkvist manages to cross over from the first category to the second.
- In the later Tales of the City books, Brian is happily married to Mary Ann, but carries on affairs with other women behind her back, only stopping after one of the other women informs him that she contracted HIV.
- In Brave New World, John has a severe MadonnaWhore Complex problem and constantly rejects his love interest Lenina's attempts to have sex with him despite his intense sexual desire for her because he treats her as a goddess of perfect purity instead of the Free-Love Future inhabitant she really is.
- On an episode of Jersey Shore, The Situation points out that Angelina regularly has sex with various men while denying sex for the man who dates her, gives her gifts, etc. The guys tell her she's a whore for cheating on the guy she's dating and that she should have sex with him because he has done so much for her.
- In Bones, we have one Ambiguous Situation episode where the titular character dates two guys while being either this trope or simply not enjoying everything about either guy - finding only one physically attractive and finding only the other to have an interesting personality. She was regularly having sex with one man while going out to museums and whatnot with the other. She enjoyed the one guy's physicality and the other's brain. The whole thing got blown when they found out about each other in a Two-Timer Date situation. Sex Guy wanted to hang out with her and Hang Out Guy wanted to have sex with her. She refused to compromise for either, so they both dumped her.
- In True Blood, Jason Stackhouse has a lot of casual sex but finds carrying on an actual relationship to be difficult.
- In one exchange from Queer as Folk (UK), Vince wonders if Cameron is now his boyfriend. Alex asks him if he thinks about him when he masturbates, Vince says no, and Alex says that means he is.
- In Conviction, ADA Brian Peluso loves ADA Christina Finn, as he admits in the final episode, but goes out of his way to be a man-whore with every other female that catches his eye.
- In The Americans, Phillip has been trained by the KGB to seduce and have sex with targets without any emotional attachments. This creates significant psychological issues for him as the Centre increases his workload, forcing him to carry on affairs with multiple women.
- In Fosse/Verdon, Bob Fosse was raped by a pair of older women when he was 13 years old. The trauma badly screwed up his sex drive, causing him to have difficulty being intimate with anyone he actually loves, which in turn leads him to constantly cheat on each of his wives.
- Buffy the Vampire Slayer/Angel: Enforced in Angel's case, since having sex with someone he actually loves will cause him to experience a moment of happiness and lose his soul. (He originally believed it to be a case of Can't Have Sex, Ever, but an empty night spent with his ex, Darla, proved that wasn't true.) In the fifth season of his solo show, he sleeps with his assistant Eve while they're both under a spell, and later has a brief relationship with Nina, but avoids growing too emotionally attached to her.
- A young Arthur desperately fell in love with a Roman woman, but was unable to marry her. He remains faithful to her by not having sex with his wife Guenievre (which causes its own problems because everyone's waiting on him to produce an heir), although he does have plenty of sex due to keeping four official mistresses around. He still doesn't produce any children though.
- Lancelot's love for Guenievre was such that he never showed interest in any other woman, romantically or even sexually. As a result, not only do most of his fellow knights think he's gay or taken a vow of chastity, but when he finally kidnaps Guenievre neither of them knows what to do.
- Euphoria: Jules doesn't really feel anything for the guys she's had sex with, and possibly the only reason she falls so hard for "Tyler" is because his supposed distance means that they are unlikely to have sex anytime soon, forcing her to develop other kinds of intimacy. On the other end of things, she hugs, kisses, cuddles, and holds hands with Rue frequently, and her ideal living arrangement is one where she and Rue fall asleep together every night, even though she never expected to feel any sexual attraction towards Rue.
- In The Bold Type, after a string of relationships with women that ended badly, Kat decides that what she needs is a relationship that's purely sex. Since there aren't many lesbians who'd be willing to put up with that, she ends up carrying on an affair with a guy, despite not being emotionally attracted to him.
- Dragon Age: Origins:
- Zevran in Dragon Age: Origins is a Professional Killer who views sex mainly as means to get close to his marks or to just blow off stress. As such, it is very easy to bed him... yet, if he really falls for you, he will go full-on Celibate Hero until he can figure out his feelings. It takes a long time for him to reconcile the idea of loving a person with that of having sex with them.
- Morrigan is a Femme Fatale whose unclear motives, dubious morality and habit of being dishonest causes everyone in the party to believe she's playing a male protagonist if he chooses to sleep with her. And she is. The issue, though, is that if the player shows her enough kindness, she actually falls in love with him and thus refuses to sleep with him from that point on. This is revealed to be a combination of her belief that Love Is a Weakness, as well as an attempt to make the protagonist hate her. Her ultimate goal was to undergo a sexual ritual with a male Warden (her preference being the protagonist, if she's in love with him) and become pregnant with the soul of an Old God. If romanced and the Warden agrees to impregnate her himself, this becomes the first time that she ever has sex for love.
- In Dragon Age II, if Hawke romances Fenris, they sleep together, and the emotional surge causes him to remember - and then immediately forget again - the memories he has repressed due to trauma. This leads him to back away from Hawke for the next few years, despite being very much in love with him/her. However, if Hawke doesn't romance either Fenris or Isabela, they get together as Friends with Benefits, and Fenris has no problems with it. It's only being in love that triggers the issue, and it takes him a few years to get past it.
- In Khaos Komix, Jamie has some major sexual hangups from being groomed and raped as a child by a man who told him he loved him. As a result, he has no issues with casual sex but is incredibly hesitant at the idea of sleeping with someone he's in a relationship with, and at one point tells his girlfriend that "I love you" is the one thing he can't stand to hear during sex.
- Go Get a Roomie!: The title character has lots and lots of sex, but has never had it with someone she loves. When she finally admits she's in love with Lillian, she's terrified of consummating their relationship.
- Martin from Ignition Zero is an aromantic heterosexual, meaning he's sexually attracted to women but romantically attracted to no one.
- In Family Guy, Spooner Street's resident pervert Quagmire is perfectly capable of having emotion-free impersonal sex with just about anything that moves and quite a few things which normally do not. He has a long-standing desire for the one woman who is unattainable to him - Lois Griffin. His infatuation for her includes passive stalking and building a shrine in her honor. On the one occasion where Lois becomes attainable - she has tired of Peter's stupid antics - she indicates her availability to Quagmire. However, his long-standing romantic infatuation combined with performance anxiety and guilt that he is somehow betraying Peter - and above all the realization he is in love with her - all combine to devastate his libido. While Peter has a last-ditch play to get her back, he is seen escalating a Viagra overdose, electric shock therapy, direct injections into his penis, and finally hitting himself very hard with a hammer. All to no avail.