You don't have to do anything tonight if you aren't ready.
You could ignore him
You could carry on the pretence it's all OK
You could go out with a friend
Fucking hell - who are you telling IRL that think 'it's not that bad'?? IMO no strings is slightly less hurtful in that there isn't any emotional connection with someone else, but the difference is minimal and certainly doesn't make it 'not that bad'.
I know what I would do now, but I know what I did when my ex Dp had an affair... I let him talk me around, I let him blame me (largely), I let him convince me he loved me & didn't love her blah blah blah - it didn't work out (clearly as he's my ex) and I would never give anyone a second chance again. You make a commitment, you break it, that's YOUR problem, not mine. Trust is hard enough for me now - fuck it up and it's over.
I wonder if we could make it work if he committed to complete honesty and change
No my love, he already trashed your wedding vows, he already trashed your agreement about how you were getting your relationship back on track. Whatever he would say to you if you talked to him about it would be just as meaningless. He's not honest and he's not commited to you - clearly - pushing him into saying he will change, wont actually make him change :(