Have you not ever had sex before?
Because generally if you have never had sex before you wont understand whats so great about it until you try it.
I'm not recommending you go and have sex since your 15.
No need to grow up to fast.
But to people who really care about each other sex is a way of expressing your love and for those minutes they are "together" and they are sharing something. I know it sounds like a cliche, but that's how my boyfriend and I both see it.
For those other people who are those guys with the popped collar and spiked hair and the girls who have skirts that look like underwear it's just something to pass the time.
Never be the person who does it "just to pass the time"
sex can be beautiful, and sex can be ugly.
When it's beautiful it means a lot more.
P.S - No asking this question does not make you a lesbian. When I was 15 I was just as curious about everything because I didn't understand a lot of things.