seems to me your premise is as misguided as that which you are trying to refute.
Mothers can kill their young who are weak so that the others may survive and it seems unlikely this can really be pleasurable or avoiding pain even though It may be necessary and instinctual and even a good idea.
I think animals do a lot of things just out of instinct… sure sex feels good and that is an incentive but doesn’t it seem more plausible that much animal sex is encouraged by pleasure… but done out of instinct to reproduce?
Though i doubt many animals understand reproduction in the same light we do they surely are driven by instinct. females reject males who are not alpha sometimes despite the possible pleasure… there must be something instinctual going on there?
Interested to know your thoughts on this