It's not unfair to not want children. He's always been honest about that. He's raising a child he never expected to have but I think it's completely understandable that he doesn't want more.
I don't know why that's seen as unreasonable. I don't want more. My husband desperately did. But once ours got past the toddler stage the thought of going back to that was a definite no-no for me - enjoyed baby stage at the time but I'm done with that (Oh so very done with that) - so I absolutely get why someone wouldn't want more - particularly given he never wanted any!
Why he's considered unreasonable for not all of sudden being ok with more kids I don't know.
I'm sure the situation feels unfair to both - it did with us but thankfully my husband respected me enough to not trick me or manipulate me and I certainly wasn't accused of being lazy for just not wanting more.
And tbh given the OP made one emotional post, referring to primal urges, I can't be sure that this wasn't an emotional response to her friend's pregnancy.
Not wanting to belittle the OP's feelings - I know how overwhelming it can be - but, even though I have no desire to have more, I am still overcome at certain times of my cycle on occasion where I'm very emotional and broody.
If I gave in to all my primal urges I'd have a football team by now and would have kicked the shit out of the dishwasher when a wheel came off the runner the other day.
That's being flippant, I know, and I don't know that this is the case with OP but I think he was right to say they'd talk about it when she was less emotional. I hope they have talked.
I can't see that he's done anything wrong here. He's allowed to not want children. I'm glad I was able to assert that right without criticism.
All that said I really really feel for OP - I do understand those feelings are overwhelming and heartbreaking. I'd say it was an almost impossible situation but given their wee family has got on thus far I really hope they can work it out.