Well incest is widely looked down upon because of the results upon the child so unless at least one partner is barren then it can hurt a person, they just aren't around yet.

Actually what really boils my blood is infidelity. If you make an oath then keep it, if you cant keep it in your pants the dont make the oath. Its that simple. It's quite possibly the most selfish thing that I can think of and there are multiple ways that it can harm your partner.