If a man is married and also dating he is cheating. If he will cheat on his current wife or partner, he will more than likely cheat on the next person as well. Unless you are seeking a permanent open relationship there is no point in dating a married man.
Yes, because one is just setting oneself up to be hurt. The best thing is to feel the attraction, find out what he feels, and then make it clear that you are going to step back and allow him to make up his min about his situation. He will let you know pretty quickly if he is ready to leave his marriage.