Ah but sex is fun, and is for fun.

It is simply not a kind of fun you enjoy with just anyone.

Indeed, what kind of fun do you enjoy with just anyone? Real fun which is meaningful is only had with people you know and already love. If not romantically as a partner, at least as friend or family or something similar.

Sex for fun is promiscuos if promiscuity is a number. But if you mean an attitude of carelessness, to me that is not so much fun as it is a wrong kind of thrill-seeking, lacking love and creating and increasing danger. To me there can be promiscuos love -- as there can be unloving exclusivity.