So you're seeing a guy and things are going great, but you've learned he's having sex with another girl and you want to know what it means. Well, what it means is that he doesn't think you and he are exclusive. Did you ever have that talk? Most guys will, given the opportunity, sleep with multiple women if the opportunity presents itself. This guy sounds like he might not be the relationship type. He might tell you something like "I've been hurt before". That's all well and good. You just have to decide what you want. Do you want a guy who is sleeping with another woman or do you want a relationship. If you want the latter, then you can make that clear in a couple of ways. The most obvious is to tell him, but this might send him running because you're telling him what to do. The best way is to accept his decision and go about your merry way, doing your thing, and not being as available as you have been. Next time he wants to come over, tell him you have plans. Do that a few times. See what he does. He'll either care or he won't and you'll know where you stand. Make it clear, you've got other guys interested in you and you're going to pursue that since he's pursuing other girls. Play his game and see how he likes it.