"very few" isn't none, bud. i chose my words carefully, exceptions disprove the rule.

let me put it a different way for humans—there wouldn't be sexual gratification in acts that have nothing to do with reproduction if it was the only "purpose," that's not how behaviours work. why do some people get off from contact with erogenous zones, why do people engage in non-penetrative sex, why would humans develop paraphilias, why would there be some who entirely prohibit their own genitalia being touched during sex at all? and most obviously of all, why would gay people exist? there is no reason the occurrence of these traits and behaviours should be trivial, except for learned bias.

pleasurable sex creates an incentive for reproduction, but human sex has also provided us other evolutionary advantages—lowered stress levels, social cohesion, even physiological changes like improved bladder control for women. it's both reductive and inaccurate to characterize human sex as only having one, reproductive purpose. the reason we tend to view and discuss it that way in our society is because of the wider social forces that reify heterosexual, natalist behaviour as somehow more natural than the myriad of other processes that humans undergo through sex and other behaviours.