It happened to us. Twice. The first time was an "accident" - meaning it was unplanned. We had been having plenty of sex with my lover for several years without protection. With that much semen - in hindsight - it was inevitable. Trust me. I'm an obstetrics nurse.
The second time was planned.
My husband not only saw it coming, but he even had deep, dark fantasies about it for years beforehand. He freaked out, and is still freaked out to a small extent whenever the reality hits a little too hard, but he knows better than to claim he was some victim in it.
Cuckolds are submissive, but they are not victims. They are perpetrators - in more ways than one. They are culpable for everything that happens in this lifestyle through their shortcomings as men and their choices as husbands.