Why does the term used for women who sleep with "a lot" of guys imply they fuck boys/men?

The term slut is used to describe a woman who sleeps with a lot of guys and is a pretty derogatory term. But why the hell does it mean "a man who has sex with a lot of women"?? It implies that a slutty woman fucks *men* and not boys. Why the hell does everyone assume that a woman who sleeps around with a lot of people is fucking real *men* and not boys?