Why have sex with someone else when you could do it with yourself?

Why have sex with someone else when you could do it with yourself?
When I was young and attending school, I learned all these stuff about evolution, genetics, etc... At the time, some of them had been proved, some of them were still theories. I still get this warm feeling every time one of these theories is proven correct... It's like "ok... scientists keep getting it right". And why is this important to me?
You see, I have no religious beliefs, but I do believe in science. I wouldn't go as far as some people that state that their religion is Science... it isn't a religion because by definition, every dogma in Science may/will someday be proven wrong, or some better explanation found for it. But it still makes me feel like I did the right choice for me (and I added the "for me" because you're free to believe whatever you want). So today life makes a little more sense than it made yesterday :)

NOW, GO HAVE SEX AND MULTIPLY OUR GENE DIVERSITY!