This one, if you need a fresher course:
Your mother puts you in a trash can when you're an infant because you're an annoying bastard. The trashman finds you and raises you as his own. He never mentions finding you in a smelly trash can.
25 years later, you meet a girl. You date, you fall in love. Turns out, your mother and father decided to try to make a better baby after you were discarded. You're now dating, and nailing, your sister. Incest is illegal.
What's the difference between you and your sis in this situation and two dudes in love, morally speaking?