Your boyfriend is the problem here, not your body. I'm so sorry that you've had to put up with this.
He's the problem because he's even doing this with you in the first place. He's the problem because he's talking with you about your body like he's some kind of expert when he doesn't know a thing about this: he doesn't even have these parts himself. He's the problem because he obviously doesn't trust you, and he's looking to things like this baloney to validate that lack of trust.
You're not going to be able to convince this guy about this: I have no doubt that given how long this has been going on for that you have probably tried and tried and tried.
So, what I'd like to talk to you about are your choices here. The way I see it, you have two main choices here. You can set a big limit with this guy that this conversation is OVER and he needs to never bring it up again (and if he likes, he could educate himself elsewhere) OR you just ditch this guy and choose someone better. I don't see continuing to put up with this as you have as an option. I mean, I guess it is, but it seems like a terrible choice to me.