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
