I asked DH last might if he was 100% sure that I wouldn't never cheat on him. After some thought he said no, but that he was 95% sure.
When I asked him why, he said that there were things that happen everyday to people which might, God Forbid, happen to us. The death of a child, for instance, or a terrible accident causing severe injury to one of us. Even redundancy, loss of our house, or other more minor misfortunes. No-one can say, with 100% certainty, how they will react to such happenings. They might, perhaps, lead to a previously abstemious person turning to drink. Or a cautious, sensible person escaping to high-octane sports. Or maybe, an absolute, dyed-in-the-wool, faithful person having an affair.
As others have said, no-one can predict the future.
No-one can truly know the depths of another person's heart and mind. To claim otherwise is beyond arrogant, almost as if their partner has insufficient will of their own.
What DH did say, however, and what I agree with, is that it wouldn't necessarily be a dealbreaker for us.
However, as with everything, one cannot be 100% sure of that, either.