Not really sure if it is as strange as people think, considering that many male humans do this in remote less developed regions, or in times of war for lack of a sexual partner. How many goat jokes do we hear during our youth... While I find any sexual contact without consent reprehensible when it comes to humans as the initiator, primates most likely do not have developed to the point of mulling over the finer points of ethics and philosophy, so they are hardly to be held in contempt for their behaviour. Fun fact it is still anchored in U.S military law that soldiers are allowed to rape a horse without repercussions - I kid you not!