@Middlelanehogger
This is the kind of crap I think a lot of people who choose to be child-free deal regularly. People who question our decisions and think they know better and that all they need to do is poke and prod at us before we come to the stunning realisation that actually we've always wanted children.
Imagine if a child-free person took the same attitude that you seem to have towards someone who wanted children. Started to poke and prod at their reasoning, trying to make them see that they really don't want children after all. There would be hell on-and rightly so-but people do this to child-free people all the time and think it's acceptable. Believe it or not, we know our own minds and if we do change our minds, it won't be because someone poked and prodded at us.
Why can't he respect their marriage and accept the fact that she doesn't want children? Why do you and he, in fact, expect her to be the one who examines her choice and makes the change? After all, it is he who is changing the dynamic of their relationship.
She has made it clear that she didn't want children. He was undecided but "fine either way." What does that suggest to you? He stated he was fine to not have kids. He's changed his mind and decided that no, kids are a dealbreaker. OP has not changed her mind.
It is fine that he has changed his mind and it's his right to leave the relationship if he is not getting what he wants from it. However, it's not fine to put it on OP and make her the bad guy. Does it sound reasonable to you that his attitude is "Give me a kid or I'll divorce you?"
Does that sound like a good basis to bring a child into the world? Through coercion? Threat? It doesn't to me. It's one to thing to acknowledge that this decision is a dealbreaker and could end their marriage if they can't agree. That's fine. It's not fine to pose it as a threat to manipulate your partner into doing what you want.
The reality is, it's obvious that he's setting OP as the fall guy for the demise of their marriage. Blatantly. He'll present her as the unreasonable one who just wouldn't, no matter what, give him the child he is so desperate for. No doubt he'll leave out the part where she's always been upfront with him about her lack of desire for kids and work it that she's denying him what he feels is rightfully his.
I am not saying that it's out of the realm of possibilities that people change their minds. Of course, they do. It's a massive decision either way. However, she hasn't changed her mind. So, she doesn't need a bunch of strangers on the internet trying to convince her that they, too, thought they would hate motherhood but they just love it so much and so will she.