disease resistance - thats why we reproduce in pairs instead of budding. there's a good episode of vice about how bananas are going to be wiped out because they are a monoculture and therefore not resistant to disease.

plus there's the whole argument that our only purpose for being is sex. there is a lot of energy in sex. it is our life energy and it can be transmuted to higher consciousness with practices like kundalini.

but personally, i just plain enjoy it :)