Because she had the choice to or not? And that goal has passed now:
They had a shared goal to become pregnant. - They both looked at options of achieving this goal and she decided she WAS willing to take the physical implications of ivf/pregnancy.
Now they both have a new shared goal - to have sex without getting pregnant.
She has said no to one method of achieving this goal - hormonal contraception.
He had said no to another method of achieving this goal - vasectomy.
Neither is WILLING to do what it takes to achieve this goal, over the physical implications.
So to answer your question, the last shared goal where she made the choice to put her body through all that, to my mind, doesn’t mean he has to forfeit his right to decide what he does with his body now.
You can’t point fingers at someone and say YOU got a baby from my body so now I get to decide that you have a vasectomy. That is forcing. And that is not ok is it?
But he definitely can’t complain if she refuses to sleep with him going forward. And he definitely should have thought his words through far more carefully and acknowledged that she did A LOT for their last shared goal and CONSIDER that maybe it’s his time to take one for the team.
And that’s the real issue here isn’t it? It’s not so much that he said no. It’s that he just shut her down immediately instead of even thinking it through - ie he didn’t show commitment to them as a team. He’s happy to ask big things of her body but doesn’t want her to ask big(ish) things of his.
although honestly, OP I’m pretty sure that’s just a man’s automatic response to someone suggesting putting a knife to his balls. I probably wouldn’t take it personally
(And before you try to paint me the villain again, I know - having had multiple kids and allll the rest of the crap we women have to put up with - that a vasectomy nothing compared to what op has been through. But men do not really comprehend that)