If he doesn't respect your boundaries the conversation stops there. That isn't the recipe for a healthy or happy long term relationship and even if you don't stay together if you reproduce you'll be forced to have a relationship forever whether you want one or not.

Edit: PS, having the self confidence to dump someone because they don't respect your boundaries comes with experiences. It's why having a child super young is often a bad idea, you don't know yourself, relationships and the like well enough to know if the person you want to have a baby with is someone you should do that with. And I'm sure there are 18 year olds here or there who have that maturity but most don't, and there is nothing wrong with not having it yet. If you couldn't get to the point of recognizing you should dump this guy on your own, you probably don't have it yourself, at least not yet.

You are 18 and only been seeing this guy for 6 months? Do you live in the US? If so you should absolutely not be trying for a baby and frankly a 37 year old who wants a baby with an 18 year old is absolutely a red flag.

And it sounds like he doesn't respect your sexual boundaries. You've told him clearly that you would prefer sex to not last as long and he has ignored that. That is another big read flag.

You are young and will have plenty of time to meet people who are better suited for you. This is does not sound like a healthy relationship at all. And I'm begging you as much as a stranger on the internet can, DO NOT HAVE A BABY WITH THIS GUY.