I guess I would want to know why he can't go. The real reason. A more healthy response is for him to want to go, to be there with his marrying friend. No "fling" should dissuade him from that.
This 'indiscretion' should not hold any power in how he & you conduct yourselves.

Tell him you really want to go, that you want to go together, that it won't make you uncomfortable, if that is his reason, then get dolled up and look like ten million bucks, hold your proud head up, dance & enjoy!

After all, you did get the guy. Tell him you would be proud to be there with him.