I'm not sure myself either. I would want to say no that I wouldn't forgive, but not sure. Just would depend on the circumstances. I mean if we were both unhappy and came to a mutual agreement on it, that's one thing. But I always told DH that if it ever came to that point. I'd tell him before anything happened. I would at least owe him that respect.
I just had a friend recently confessed to me that she "cheated" on her husband. She went out with some friends and one of the guys kissed her!! She said that he knew she was married, but was mad because she got married and he didn't have a chance with her(which is bs because for almost three yrs she was single and he didn't want to be anymore that just"friends"). So, he thought if he did this, she'd tell her hubby and he'd leave her. I asked her if she kissed back and she said no. It was one of those caught off guard moments. I told her I personally wouldn't have told my hubby(at least I don't think) because her and that guy are really good friends, she said that he was waaaayyy drunk and didn't even remember what had happened until one of his boys told him what he did. her Hubby has only met him once or twice and likes the guy. As a matter of fact, they just went and hung out all together not that long ago and she said that they all had a good time.
IDK. If this was me, it would depend on just how serious that kiss was and if I was into it too!! If the answer would be yes, then I'd tell my husband. But, if it was just an isolated incident, then no. What's the point of getting into an argument about something if it meant nothing?? It would do more harm than good by telling, in that case. I've seen men say things to my mom and women to my dad and they didn't tell each other. Why?!?! Because it was stupid and not worth it!! If or when it became a problem, then they said something, which is probably what I would do.
I'm sure I'm going to get alot of flack for this, but this is MHO. Now as far as a straight up one nighter, ooohhh heck nnnooo!!! His bags would be packed and out the door!! I've kicked him out over alot less stupider stuff!!!!

But, that's cause we were fighting and I let come back about an hour later!!!
But, seriously, if it involved sex, NO WAY would I forgive because what if he got her pregnant, she gave him something, fatal attraction, etc?!?!? That's too much of a headache to deal with!!