This is sexual assault. If you say stop and he doesn't, it's assault. You need to stop seeing this person immediately.

A 37yo man has no business being with an 18 yo. He's with you because you are young and naive and can be persuaded that this is normal. A woman closer to his age would never take this abuse from someone. He's abusive. He says he wants a baby so that he can trap you and sell you on the idea that you'll be a family for ever.

I assume your parents don't know about this guy. If you trust them, tell them about this guy.

In any event, please refrain from dating anyone over 21 at this point. Better - no dating until you figure out how you got yourself in this situation and why your self esteem is not stronger.