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?