I'm not sure what you don't "get". It sounds like you and your husband are at an impasse. he does not want any more children. You do.
He is not willing to have "permanent" birth control. Lots of men are like this. You need to discuss birth control options with your GYN.
It sounds like you had a talk where you asked if he wanted more children and his response was to not answer you. You, therefore, sat in silence for 30 minutes until you left the room... correct? That is not a conversation. And he is telling you in more than one way that he does not want more children.
Further..... sex and procreation (or making babies) are two completely different things (unless you all are devout catholics, I guess). Sex is about intimacy and connection, physical pleasure and release, and getting closer to your partner. Don't confuse the two. And don't "withhold" sex because he doesn't want to have another child. That's not healthy (plus.... then you don't get any, either).
As far as giving him an ultimatum..... what would it be? That he must change his mind and have a child or you will leave him? You are correct... that is not an ultimatum you should give. You should explore why he doesn't want to have another child and why you do. Many times this is not an issue that can be 'compromised' and typically the NO vote is the winner.... you should NEVER bring a child into a world if they are not wanted. Instead, accept his position and find ways to fulfill yourself. If your MIL is not loving... then find friends who ARE loving and who share your interests.
I would suggest counseling. It sounds like you all are not on the same page. If he won't go.... you could go alone. That might help you, at least.
Also - on additional note of perspective. If your husband's MOTHER has never been loving.... it's likely that he doesn't really understand the concept or how to receive love. There is a reason it is a "cycle". we only do what we know.