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.