Sorry to break the bad news to you but in a word, yes. This man sounds like a "serial cheater" and although you may think it wouldn't happen to you, it will. Some men collect women and affairs like trophies and the best predictor of future behavior is current behavior. There is also the possibility that he is a sex addict and loves the "thrill of the hunt". If he cheats on his wife with you then chances are when/if you become the new wife, the pattern will repeat itself. Guaranteed. Remember, you cannot change anyone. They have to want to change. This man needs counselling to understand WHY he has the need to go outside any relationship for sex and cannot stay monogomous. If he exhibits sociopathic behavior of which lying is a big component then my advice is to stear clear. Odds are you will never change him because he lacks the social component to see that his actions are harmful to you and anyone he comes in contact with. A sociopath cannot tell the truth, even to himself and relationship potential is zero. You will never change him or the behavior. Sometimes we have to fall before we can walk and you may need to learn this for yourself. Be kind to yourself and steer clear. He may have his eye on you now but the "other eye" is always looking....
Good luck