The character is emotionally unable to love and have sex with the same person.
Rolling Updates * Needs More Examples
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 fomerly popularly called the Madonna-Whore 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. 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.