(I'm 16, but I know I'm gonna sound like such an old man here, haha)
Really, it's because sex is a serious commitment. Especially for the girl. It's not just physical.. It's emotional. You're giving a part of your heart to the boy you're having sex with, and what happens if it doesn't work out?
Plus, teen pregnancy, and STDs are on the rise, all because of stupid teens who didn't think twice.
It doesn't matter how nice you are. It doesn't matter how much you *loooove* the guy. The reality is that kids aren't emotionally ready to handle sex and it's consequences.
Plenty of really nice, really 'smart' girls give it up too early. That doesn't make you any less of a person. It just makes you look like a stupid kid to people who are older and wiser than us, that's all.