You say it's lose/lose, I'm not sure that's true.
You're assuming that if you leave him your life will be as "unfulfilled" as if you'd stayed with him and "agreed" to stay childless.
Frankly I think the only loss is to stay.
I'd rather live a life alone on my own terms than regret living one together on someone else's.
A relationship tainted by resentment (and I think you'd end up feeling this way) is far worse than being single or in childless relationship with a different positive dynamic.
It's not wrong for your DP to not want more children. Neither is it wrong for you to want to have a child of your own.
The key thing here is it's you making all the sacrifices and taking all the risk.
You assume you are swapping being childless for a fabulous "until we die" commitment.
The scales are not exactly balanced here are they? He's got his kids and a younger partner. He's your BF so not not even committed enough to marry.
If you spilt up in the future who has invested the most in this relationship and stands to lose the most....he will be fine, no maintenance or child support; you having sacrificed your fertility.
It's not really a partnership is it? - be honest to yourself.
He's got what he wants and yes that's fine and dandy. If he doesn't want more children the maybe he should date older women with children? Or women who don't want children?
I frankly can't imagine "feeling the love" for a man who was happy to be in a relationship where he expects such sacrifice from someone he purportedly loves.