What do you do when your fiance says he doesn't know why he cheats and why he is tempted to cheat and you satisfy him to the fullest but he has a serious problem that he doesn't know how to fix?

He has to be the one to acknowledge his problem and seek help (therapy) for it. Getting married will not stop him from cheating; instead he will take his affairs even more underground. Unfortunately, no matter how much you love this man and how great your sex life is, you cannot expect a monagamous relationship with a serial cheater. It's *not* a lack on your part, emotionally or sexually. It's about what he feels he needs to fulfill himself, a bottomless pit. He may love you and be generally content in your relationship, but it won't satisfy his need for the rush, the ego satisfaction he gets from the 'seduction' phase of a new affair. For him there is something addictive about pursuing and seducing someone and also being seduced. This is especially true knowing that he's in an otherwise secure relationship where if an affair ends, then no big deal--he has you to help fill his needs as well. It's better to know now what you can expect if you stay with him, rather than find out ten years down the road when you have a shared life and possibly even children.