I think it's critical for you both to be in the same building and meet it head on. The thought of it will haunt you for a while however you face it, but you are married with a new born and (I suspect) love each other very much (apart from this, which will shake you).
I only say this because your child does need both parents there at present, if there's a chance you will remain together, it's important that you are both 'the norm' right now. (If it's over, your baby will also be fine, but that's a different norm!).
If a man loves you, your body can't be beat. That doesn't mean he won't find another unattractive. It's you he wants. He's ALMOST denied himself that forver via his own stupidity. My lovely DH has an ideal 'body' and it isn't mine, but it's mine he wants... does that makes sense? You need to talk, you need to be face to face, you need to have him in the marital home and work through it. You may not want it, it may be painful, but this is your marriage... if it doesn't work out, you've given it your best.
I'm not saying this because I'm liberal or an ice maiden who can simply put aside bad, hurtful, stupid behaviour, but because maybe our 'golden oldies' have it right, it's between you two, you have to face it, or you'll have no chance. You have the moral highground here, hopefully he'll be feeling distressed and scared too. Do not give MIL (no matter how nice) TMI... as a couple, if you're to see this through, you need to face each other sooner rather than later.
No second chances, lots of talking, lots of tears, united with your baby, and a lot of time. Rash decisions will see you single quick, and that is OKAY if that's the way it turns out, and it will be his fault ultimately. My advice is be brave, tell him all of this again and again and again face to face, just you two, and see where you are in 6 months (there is no quick fix). x