I'm very much in accord with what s4ndm4n says above. These are words of wisdom, IMHO.

Right now, winning you back is the challenge that he is kind of enjoying tackling. Very possibly, he does really want this marriage to last. That doesn't mean he truly wants to commit to monogamy. There are men who totally want a reliable wife at home, while they partake of sampling the occasional other sexual opportunities that come along. What most of us fail to understand is that, deep down in their souls, these kind of men (or women) really don't consider what they do to be all that wrong. That is the reason why change over the long haul is so unlikely. As one comedian has put it, "Some men are only as faithful as their options." Right now, there just doesn't happen to be someone flitting around his periphery that he'ld like to get a "sample" of. Eventually, opportunity will present itself again.

He truly may have zero intention of ever leaving you. And he may think that should be good enough for you. People tend to not do things that they believe are very wrong. They certainly don't do them repeatedly. When they do, you need to believe what they are telling you - that, in their mind, this is not all that wrong. You can't bridge that gulf. The two of you hold different values. He probably can't be converted to your idea of marital commitment. Ask yourself what was role-modeled for him by his father. It might tell you something. (or not . . . IDK, for sure.)

You're trying to rationalize yourself into believing what that little smart part of you just isn't buying. Having said all that . . . you don't have to walk out the door tonight. You may need a bit more crap to happen before you will really have had all you're going to take. But, in line with what's offered in the post above, start developing an "exit plan" for "just in case."