Try this. (And keep in mind that it's from back when I was single, and thus younger and stupider.)

If we're friends, what's wrong with friends having sex occasionaly? And just because you start having sex, why do you have to redfine the "relationship" to something else? Dating, going steady, boyfriend/girlfriend, partner ... women are the ones always asking, "What are we to each other?"

For guys it's friends I can sleep with / friends I can't sleep with.

Why are you friends with someone? Some it's because you both like sports; some because you like the same movies; some because you can sleep with them, or at least think there's a chance you might be able to. If the only thing you ever matched on was the sex then no, you won't be friends when you break up/stop having sex. If you connected on other things, then you can still be friends.