I agree. All we truly have is our honor and our word. If I was in a committed relationship and found out that my S.O. was unfaithful, my trust would be just as destroyed as if I were married.
FWIW, I would wonder about a guy who was celibate for 10 years. I would think in that time he would have found what he was looking for. But, if I did decide to get involved, I would easily call for some sort of comment either way by 6 months. You might not know if a given person was someone you could marry, but you would know for sure it this person wasn't right. And I would be very upfront with him that if we anre't in a committed relationship leading to marriage, that I would date others if I decided to.
It's also almost impossible to rebuild that trust. Sometimes you can be very careful and not know what you're getting into. Sometimes people are darn clever about misrepresenting themselves. These days you never know...