Are we talking about affairs or sex?
I don't think affairs are excusible. (literally full stop)
However my then boyfriend, now husband, had a one night stand and told me a day later. We had been together over a year and had just come back off holiday. I still feel angry but more so with the woman as I could never do that to another woman. She knew me also that OH and I were in a serious relationship. I feel she deliberately went out to seduce him and not to put my husband down (literally) but he is a man and was a little niave as to womens intentions ie interprets outrageous flirting as "just being friendly".
Basically I don't think I could help a man make a fool, intentionally or not, out of another woman.
If I were that attracted to another man then I would have to question the relationship I was in.
I love my husband only fractionally less than my dogs but if he had an affair then I make John Wayne Bobbit look all man.......