I understand your concern, and while I don't think you should put on blinders and be a sucker, I don't think you have the right to determine how he is going to live his life.
Really, i think you need to take a step back and ask yourself honestly if this guy makes choices you can accept (and I don't mean "tolerate"). You have to decide whether or not you will trust him (and again, I don't know whether he is trustworthy or not). If you feel there is a high possibility he will betray you, then break off with him and find someone you CAN trust. Life is too short to spend all your time worrying.