Hmmmmm... I don't think I'd tell the cheater's spouse if all I saw was kissing (even passionate kissing), because maybe the person just had a momentary lapse of judgement, smooched the other person, then pulled back & snapped out of it & felt crummy & will never do it again! I would say something to the Guilty Smoocher, along the lines of "Hey-hey-hey -- what the heck are you doing?" but I would not tell Guilty Smoocher's spouse.

Now, in the case of inappropriate touching or, er, heading on into the No-Tell Motel, then I agree with those of you who said you'd confront the cheater & give then 24 hours to tell the spouse (or else I will).

I am not advocating random make-out sessions for married people -- lol -- I'm just not ready to scream "adultery!" about one kiss... It could be just one of those good-bye pecks that the other person suddenly turned into, er, smooch-fest!