Most animals will only engage in sexual activity that can result in reproduction. There are some exceptions; Bonobos, for instance, engage in a lot of sexual activity that does not involve coitus.
Human beings have a large range of sexual activities and fetishes that do not involves exual intercourse that can result in pregnancy (I am not referring to the use of birth control).
Oral sex, anal sex, and a variety of fetishes that do not involve coitus between a human male and female.
Given that the origin of sexual acts pertained to reproduction, somewhere along the line there has been development in the brain of humans and some animals, to include sexual activity for pleasure and acts that have no possibility of impregnating the female.
It can be argued why and what purpose these acts have, whether they are indeed part of the mating process. I am not interested in this so much as, the development in the brain which has offered this variation in sexual activity to some species.
This is not at all a social-psychological question, but a question focusing on evolution, embryology and the brain. From a macro to microscopic view, if applicable.
What is the difference in the brains for animals capable of these great differences in sexual activity and what part of the brain is responsible for this?
I am not assuming that all sexual activity involves pleasure. Dogs are known to dry mount other dogs in an assertion of power. So there does appear to be a development throughout evolution, as to the use of sexual activity and communicate and purpose. Definitely, human beings have a wide range of pleasurable sexual activity derived from non-vaginal intercourse, that the vast majority of species do not. It could be argues to be counter evolutionary for want of a better term, as, like much human behavior, it serves no purpose in the continuation of the species.. OR maybe it does. Either way, it is an interesting advent of behavior.