That depends on the situation. My wife kissed another girl, but it was in front of me and it was a sort of dare type situation. Although I feel like they both enjoyed it more than they should have.
It depends on why they did it. My friend and I kissed in clubbing once in front of her boyfriend because some guy wouldn't leave us alone and didn't believe we were 'lesbians'. :\
I'm straight and my boyfriend said he wouldn't care if I kissed another girl. However, if I turned bisexual, then he'd probably have a different opinion on it and would count it as cheating.
No, not really. However, though, I suppose a guy would get jealous if too much time was spent with a woman. Anyway, on the lighter side, I suppose the boyfriend would get turned on.