Newsflash: people have sex because it feels good and they're horny.
LOL.
I love the list of 237 reasons why people have sex, which keeps expanding as people offer more reasons. I think a lot of them ("It was an initiation rite for a club or organization," "I wanted to give someone an STD") are basically the result of people trying to invent any conceivable scenario why someone would have sex.
"I was promised a cake in return."
"Can only do activities beginning with S."
"Tripped and fell on a penis, decided to just go with it."
"Court order."
Invent your own!