I took this picture in Borneo, years ago. They are Proboscis monkeys 
In my life, in my travels, I’ve observed primates engaging in NON-REPRODUCTIVE SEXUAL BEHAVIOURS many times. I’ve witnessed different species of monkeys masturbating, stimulating others, engaging in homosexual behaviours, non-copulatory mounting, oral sex, and sex with juveniles (i.e. not fertile). Other forms of non-procreative sexual activities can also include genito-genital rubbing between adult females, anal sex, inter-species mating, sex with pregnant females and sex with dead animals (necrophilia).
So, why do primates (and some other animal species too) engage in sexual activities that do not lead to the reproduction of the species?
To solve disagreements and relieve their aggression with each other (especially bonobos)
To create bonds and interact with each other
To demonstrate their dominance
For “trading purposes” => Female of Adèlie penguins can trade sex for stones, used to build their nests, and female chimpanzees can offer sex to males in exchange for meat.
To stimulate erogenous zones and therefore feel pleasure!
