I worry about my best friend being too sexually active
Oh, my. Why do you think they're too sexually active?
He has lots of girlfriends, sometimes more than one at a time.
You care about your friend's and it shows, but the friend needs to be more careful. You should have a talk about this and be straight with it all. If you don't care about it then they will end up with an STD or something worse.
My problem is that he isn't learning his mistakes from his past experience. I have to keep looking out for my friend.
If your friend truly won't help himself, then it may be time to move one from him. Cutting him off may help your own mental health.