I sympathise massively op, i've known since i was very young i never wanted children. Sure i played with dolls, but i began suffering from OCD aged 5, all about toilets, using the toilet, bodily fluid germs etc. It got worse as i got older, and before i hit puberty i knew i'd never cope with a baby and i hated the thought of having one. My biggest fear is incontinence, i could never cope with it, i couldn't handle peeing every half hour (with OCD it takes me 45 mins to go) or the baby kicking my bladder and causing me to wet myself, or the post birth "accidents" when sneezing/running/jumping etc. Nor could i handle having to change nappies, the wees/poos they do mid change that go everywhere, nappy leaks, potty training, bed wetting, the fact they stick their hands down their own nappies, and have no sense of clean and dirty and will touch anything, and not being able to control if they wash their hands after the toilet once they get older.
Stretchmarks i have anyway due to rapid weight gain, i'm already overweight, and tired all the time from the constant rituals and stress of having an anxiety disorder. Having a child would be a literal living nightmare for me. IF i hadn't developed OCD maybe i would feel differently, but i don't feel robbed of the chance to be a mum because i honestly have never felt any desire to be one.
I get that it's a very rare thing though, something like 85% of women have at least 1 child in their lifetime, and the 15% that don't consists of those who physically can't have children, those who were capable but left it too late, and those who just don't want them.
Every woman i know who has children considers them to be her greatest achievement in life. They're her whole world, the best thing she ever did, had always wanted kids, she adores them, and many can't wait for grand children. It must be so alien to them that some women don't want children. That how adamant they were that they wanted children, someone else feels just as adamantly about NOT having children.
It's not easy, they think something is "wrong" with you, so many people see the point of life is to have a family, they just can't see any other opinion. It's incredibly brave of you to be so open about how you feel, and your husband went in to your relationship and subsequent marriage knowing you might not have a child at all, and that IF you did, it was an absolute maximum of two, with the second only even considered if you enjoyed motherhood and were willing to go through pregnancy and child birth again, which you aren't.
He went in to this arrangement with his eyes wide open. Expecting you to have another child you do not want is unfair, he knew this scenario was a possibility, and the fact he's begging you to not go for sterilisation means he isn't respecting your feelings and hopes to change your mind, wear you down etc to have another child. You love each other, but he needs to also respect you, your body, and your decision that a second child is something you just don't want. I wouldn't leave him for the sake of giving him the chance of having a family with someone else, there's no saying he'd find someone he loved and wanted children with, or that they were able to conceive, or that their relationship wouldn't fall apart. He could end up being a part time dad to your son and that's it, worse than the situation hes in now. I would however, sit him down, make it perfectly clear that you will not be having any more children, and that you're booking for sterilisation. You respect he will be upset, and its not what he wanted, but he also needs to respect that it needs BOTH of you to want another child, and you just don't. If he still tries to pressure you, guilt trip you etc, i'd leave. If you stay together and further down the line he starts taking it out on you, treating you badly etc, again i'd leave. I'd refuse to feel guilty about not wanting to grow, birth and care for another baby, another life, as it is the biggest decision there is for a person and it isn't right for you.