A female character tries to seduce a male one, who doesn't want to be seduced. He rebuffs her advances by simply saying "Sorry, I'm gay", regardless of his true sexuality. This trope is meant to be gender-neutral but for some reason, "Sorry, I'm lesbian" doesn't work just as well. (Not that "Sorry, I'm gay" works all of the time for men either.) It does work sometimes, though.
A subtrope of [...] [tvtropes.org]