I would say you guys are doomed, if you can't get over this. I can tell you from personal experience, you're going to resent her for it forever, most likely. Bang her in the ass, then bail out.
It's easy to say that what someone did in the past didn't matter, that it's her body and her choice, or that she didn't know you at that point, so it's meaningless now because she's with you... But, really, she still let that guy slide his dick in her ass. So willingly, just to get him to like her. You'll always know that. You can try, with time, to convince yourself that you don't care, but you always will. ANd even though you might be able to put on a happy face inside, deep down you still begrudge her of it, and ultimately it's not fair to you or to her to have to force yourself to be happy about something like that.