How about looking at this from a different angle? Tune out the romantic phrases that run through our minds because of our cultural conditioning, and try to be detached and scientific about this.
There are all sorts of relationships that we have with people. Right now you have, what sounds like, a very good co-habiting and co-parenting relationship. You’re probably good friends.
If you don’t marry your options are to continue to cohabit, or to move to two separate homes.
If you are going to continue to cohabit, marriage might be a sensible option, because it provides legal benefits, clear next of kin arrangements, inheritance pathways etc. Or it may not be. But base your decisions on the fine print of the marriage contract rather than on fairytales.
If you decide to part ways, you have the basis of a solid co parenting relationship and that is an excellent achievement. There’s absolutely no shame in choosing this type of family arrangement if it meets your needs. People will talk for a while until they get bored and move on to the next drama, particularly if you refuse to be drawn. You get one life, don’t waste it.
He’ll be shocked and hurt at first, but right now it sounds like you’re carrying all the pain of this situation and that’s not fair either. He’ll also get over it in time.
I would do some hard thinking and then pre empt any showy proposal. If you need to buy more time to think you could drop some comments about how cringe public proposals are (although I wouldn’t rely on him to hear you as he already has firm for ignoring the obvious).
Has it ever occurred to you that the ex might have lied? If she went through a similar wow to nothing with sex, she might have wanted to hurt you.