well if you follow darwin's theory of natural selection, more
good genes would be passed on to the next generation through sexual
reproduction, and so on and so forth, so over time, the gene pool
for humans (or any other animal that reproduces sexually for that
matter) would have better characteristics.
eg. a fit and healthy guy or gal would be more likely
to..ahem..have sex than someone who is fat and unfit (these
characteristics might be genetic) so more ppl of e next generation
would inherit the gene to be healthy, and so on and so forth, to ad
infinitum (this is an extremely simplistic representation, forgive
me)
well..feeling good during sex due to orgasms or otherwise are
prolly an incentive for humans to sexually reproduce. otherwise if
there were no uh..good feelings while having sex, would people have
it in the first place? so whoever designed this system..it sure
does work 
