Using the scenario my wife finally confessed to, factors came into play. Depressive and stress of homelife, discontent with me, (this is the night after “happily” celebrating our first double-digit wed anni?) She knew the guy back when she was 17, before meeting me. He certainly was no “upgrade”, but he was married also. I’m sure there was the spouse talk and what they were unhappy with, by this point she is 4 drinks in at least, defenses are down, he finds the moment to lean in and kiss her and she doesn’t fight it.