Because if a guy has love when he's young, he scores.
If a girl has love when she's young, she's a whore.
At least according to the media or what other people say.
My theory of why society looks at it like that is this:
It's an evolutionary psychology thing. Think about it like the following (not using humans as the example species here, rather just a general one)
The male of the species has an inherent instinct to spread his genes as much as possible, which is why that's a "score" or good thing if he does
The female of the species should and needs to carefully select a mate, risking mixing with the wrong genes if she doesn't, so if she sleeps with lots of males, it's a bad thing; whore.
