This question falls into the "could fish discover water" class of questions. Another words, being human, this is such an internal part of our existence we may be ill-equipped to answer it. As humans we fall prey to seeking direct cause for everything we experience where much of our existence is due to things that just are. Bear in mind nature (i.e. evolutionary forces)is/are not interested in making dramatic difference unless absolutely mandatory; only enough difference to address the need at hand. Ontogeny makes clear we are all cut from a prototypical human form. One where small changes in genetics and resultant hormones bend that form one direction or the other. At the most basic level this means men posses redundant breast tissue and women to a greater or lesser extend retain orgasm. Both are residues of our development but not essential to continue the species. In a similar manner, we also retain rudimentary sexual functions of other species such as pheromone receptors.

Concerning humans certainly there are greater complexities. Particularly when we consider that humans are the only species on the planet where the state of fertility is nearly hidden from women and totally opaque to men. Further human infants are born in an immature state compared to other species which requires prolonged parenting to successfully transmit our genome to future generations. Consequently perpetuating bonding systems to assure successful conception and child rearing to a state of independence will be more pronounced in humans than in other species. In this, female orgasm may play an adaptive role in recruiting and retaining males to provide for infant survival.