1. Wasting your time
If you think that this guy is gonna leave his wife for you, chances of that are slim. After all, if he wanted to be with you then he would have already left her and not put the both of you in this situation. In the end you will most likely be wasting your time.
2. No self respect
A woman with self respect will not allow herself to sleep around with a man that has made a commitment of loyalty to his current partner. This just shows that you do not have self respect for yourself or others.
3. You're helping break a marriage
Breaking up a marriage is never going to lay well in the eyes of anyone else, nor your mind. You will always know in the back of your mind that you facilitated breaking up of a family.
It's true, if they cheated with you, they may cheat on you. Even if the two of you end up together, you will always wonder what he is up to and what sort of morals he harbors.
5. NO man wants a woman like that
Realistically, no man wants a woman that sleeps around with married men and facilitates cheating. It says a lot about that person and their character.