I don't think I'm going to let my boyfriend sleep with me.
Why won't you sleep with your boyfriend anymore?
Because he keeps talking about previous sexual experiences.
Well, if you're that upset about it, then you're well within your rights to not sleep with him, even if he is your SO.  I think you two should probably talk through this, however.
I don't care if it's within my rights to do this.
It is within your right to withhold sex from your boyfriend, but wouldn't you rather work this out with him? If you're having a problem with him, then denying him sex isn't going to resolve the problem, it's only going to make him resentful. Sit down with your boyfriend and have a talk about what's bothering you.