Ok, here is the situation. You meet someone, fall in love, grow to adore them with all your heart, and eventually get married. After which, your new spouse and your opposite sex parent confess that they had a one night stand before you and your partner met. It was in a bar, no emotional attachment, just a spur-of-the-moment act. When you reintroduced the two they didn't immediately recognize each other, and once they finally put the pieces together, you were too far into the relationship to make such an admission without great damage. Still, the awkwardness and guilt have laid on them and grown since your recent marriage, to the point where they felt they could no longer keep it a secret. Neither your parent or lover has technically done anything wrong. What do you do? Do you stay with them? Never talk to your parent again? Could you handle it?
By the way, this question is in no way based on a real situation. It is simply a really complex hypothetical dilemma, that I am curious how people would react to.