The reasons not to get involved with a married man are numerous - it's a really bad idea and something that no sensible woman should ever do. It will only lead to heartbreak and can devastate not just your own life, but that of his family as well. Getting involved with a married man has huge potential to hurt everyone involved. So here are some sound reasons not to date a married man.
Of all the reasons not to get involved with a married man, the primary one has to be this: it's just plain wrong. Both a married man and his girlfriend can come up with all sorts of reasons to justify what they're doing - it's real love, his wife doesn't treat him right, etc. Those are excuses - cheating is never right, and a decent person just wouldn't be part of it.