Really, they can't figure out the answer to that question? It's simple and I learned it in a sociology class decades ago:
Sex is fun in order to make sure we have sex often, thereby keeping the human race in existence. We are hard-wired to enjoy it! Thank you evolution!
[And for those of you who don't believe in evolution, feel free to stop having any sex with anyone and anything and kindly disappear as an example of Darwinism.]
If sex was a chore, equivalent to cleaning a particularly nasty toilet, how often would you do it?