There's no reason why the word shouldn't be used. Dictionary.com's definition of the word reads:
|1.||characterized by or involving indiscriminate mingling or association, esp. having sexual relations with a number of partners on a casual basis.|
Surely, then, a boy who sleeps around is also promiscuous?
The term I sometimes hear applied, however, is "stud". This is the male equivalent of "slut", because apparently if you have a penis and you sleep around, that's fantastic and there's nothing at all wrong with what you are doing. It's supposed to make you feel good about yourself, too. I don't know what I'd do if somebody called me "stud"; it's like being told you're the bump on the bottom of a football boot, and I don't like football.
If you're a girl, however, and you've held hands with more than one person, you're the root of all evil and that's why burning at the stake was invented (the wizard stuff was just hocum, propaganda spread by the government spreads easily...). Another old friend I probably haven't mentioned was labelled a "slag" (I've eliminated the 73 million extra "A"s the LiveJournal commenter felt he needed to add until everyone found out that she'd slept with seven people. It's more than one, but it's hardly a shedload. Unless it's a very small shed.
So, what do I call someone who's slept around relentlessly and is a boy? What do I call the guy who had half of the first year at university? ("Cunt" was the word I decided upon; it's actually not a bad description, either.) I can hardly just go up to a boy and say, "you are promiscuous." It conjures up an image of a girl in a short skirt with very low morals. That's just somewhat unfair.
I think it should be used more often. Boys, admit your promiscuity. It's only a polysyllabic word, after all. It's not going to hurt anyone.