I really don't know anything about it, but I'd speculate that the fantasy is plain old 'misattribution of arousal'. The heart gets pounding at the idea of violating the taboo against incest, and that bodily state is interpreted as sexual arousal. Not that I'm suggesting that there is just one explanation of something as complicated as this.
you are both correct, but what if there is no trauma?
two teenagers move into the same house because their parents started a relationship. they see each other little because both step-parents have shared custody arrangements. they have no biological connections whatsoever. four years later they are both 19, alone together a lot, et cetera. but simultaneously, this is a huge taboo, a betrayal of the parents, a great psychodrama
i knew one guy in college whose girlfriend cheated on him, with his biological father. a great betrayal, a great freudian cuckolding. what kind of strange fantasy will this man have now?
