Sex is a human instinct. It is pleasureable, and its how we reproduce.
It's the best thing there is on this earth, and for some its the only thing worth living for.
Everyone has sex, whether they admit it or not. Some people have sex with other people, and some people have sex with themselves (thats called masturbation!)
But in the end, everyone has sex of some sort, unless they've been brainwashed by the media or a religion. It's entirely natural, and not gross at all.