I think you should of just asked the question of why people will cheat. This is not something only men do and it should not be viewed this way. Some men do cheat on their wife/girlfriend/boyfriend, yes, but is it fair to say most men do it?
As for my men cheat, maybe some of them find it to be exciting. Maybe they have mates who treat them badly and they search for someone who will love them back, while they still have feelings for their wife or girlfriend who they don't want to lose.