why do people have sex

All kinds of reasons. Enjoyment, pleasure, to make babies, to share something very intimate with their partner, because they're horny, because they're inebriated and it seems like a good idea at the time.

This is probably the funniest question I've ever heard.

Because it feels good (or so they say) and because it's the most common way of getting a child. Sperm banks are available, so no one really has to have sex anymore to get pregnant- but if you're gonna go thru all that pain of having a kid, I say might as well have some fun first, right? :P

To kill a few seconds before the light changes back to green.

Cause they're horny :)

Becasuse They Want To Spice Up Their Love Life!