Christian teen/ sex dilema?

Gosh I'm 18 but I want to have sex. I've done it three times already. I've repented, but somehow found myself tempted and giving in again. I am A Christian, and I love God, but I was just wondering, why is it that anything and everything that I (and alot of other teens) want to do is wrong -Why? Theres nothing fun about it :( Please help me.