Humans and dolphins are the only animals that have sex for pleasure.
We're also two of the smartest... maybe we're the only creatures who realized that it felt nice. xD
But yeah, sex isn't just for reproduction... If it was, why was birth control invented? I think people should be smart (SAFE!) about it unless they're trying to get pregnant, but it's actually a fun activity too. =P
I've said too much. XD But really and truly, it's all our instincts and hormones. I actually don't like how some kids now are learning that "ALL SEX IS EVILLL! YOU WILL ROT IN HELL!" Honestly, it's all bullcrap. Sex should be a way of expressing your love. Love doesn't equal evil.