Most Helpful Guy
Exes that sleep together is sort of a weird territory. You're clearly not dating one another, but you're also not strictly fuck buddies. You do, however, have a shared history and some form of friendship. His talking about that kind of thing certainly seems to imply he's thinking of you more in terms of friendship than courtship at this point. If that's not where you see this going, then you should be up-front about that and be ready to move on.
Most Helpful Girl
He is your ex so you can't really confront him about that. If he brings this topic up again just tell him you're uncomfortable.