Let's say you had the opportunity to do so in these situations.

1) He was willing (maybe he even wanted to watch). 2) He didn't know about it but you suspected she might have had a SO. 3) She wanted your baby instead of his (but you knew she would make him think it's his because he was say, far more well-off than you).

I would on all three counts. Why? Because manginas deserve what they get (for being enablers) and I'm the kind of guy who kind of likes fucking and doesn't mind spreading his DNA, especially if he doesn't have to pay for it. How about you? Please explain your reasons.