DD, I agree with what you've said above, but don't look at making an ultimatum as quitting on your marriage. I could easily argue that standing by while your W has an affair is quitting on your marriage and that leaving her "for now" is actually fighting for it. If you believe that marriage is two people joined before God and committed to each other, then tolerating a marriage that is NOT that is disrespecting marriage right? Sometimes to get what you want you have to first go the other way, and this is one of those times. Not telling you to leave her, but you must do something to protect yourself and bring a stop to the cake-eating.
Speaking only for myself, the way I finally looked at it was a matter of STANDING UP FOR HOLINESS in my marriage. I gave it a great 3-month fight, that -- with God's grace and help -- was ultimately successful. But at the 2-month mark, after 60 days of post-confrontation, post-exposure, unrepentant adultery . . . I made the very sad and difficult decision to file for divorce and full custody of our two minor sons.
Hardest thing I ever had to do, and I had several reasons for doing so. But the #1 reason was standing for holiness in my marriage.
That's a spiritual belief and I am NOT telling you what you need to decide for YOURSELF, DD, but only pointing out as the others did here that there is more than one way to decide the whole "God" thing when it comes to divorce and infidelity.