I'm tired if texting dirty with my bf..

I am tired of texting dirty with my bf! Don't get me wrong I love to do it, but he takes it too far. He starts to bring up a scenario of me having sex with someone else. And this someone else HAS to have a huge one! I don't like that scenario at all! I've already told him not to bring it up and he doesn't listen to me. Does anyone else have this problem? ? I NEED advice.