Whatever the problem is, I suspect that the damage is unfixable. I'd just not spend anytime with them and see if you do okey with other people. If you do okey with other people, then they may just be defensive about their own behavior which sounds inappropriate to me. If you have problems with other people, you might consider doing some soul searching. However, I suspect that you will find that it is them.
As for you being interested in a guy with a girlfriend, I find myself being a bit against that. However, it would depend on how long it went on. Also, it would matter if you did anything with him. Besides, we all screw up sometimes. If the guy is a "womanizer" then he probably runs around more than just having an interest in other women. So, my main comment is the damage is done. You can't change things now. So, learn what you can from the experience and move on. I hope you work things out in your own mind and manage to continue to see yourself as a worthwhile human being even if you might have made a mistake.