Actually there are very few primates that don't have sex for pleasure. Studies performed on most higher mammals show that the majority of social mammals( wolfs, dogs, big cats, hyennas, chimps, etc.) have not only well developed clitori, but use sex much the same way humans do. Really people, when was the last time you had sex to reproduce? One theory is that orgasms are necessary for good psychological health. They also help establish stronger bonds between members of the group.