I would argue that sex is different because, looking at the animal kingdom, the result is rarely a benefit to the individual. It’s resource intensive and in some cases (with insects specifically) the ushering in of a new generation results in the death of the parent. Reproduction is absolutely essential to the survival of the species but often a bad deal for the organisms involved. It’s a bodily function that many resources are dedicated to but is the least essential for for the survival of the individual.

Part of what enables pleasure-sex in humans is the fact that penetration does not trigger ovulation and that happens on its own schedule so evolution selected for mating strategies that would lead to frequent and short encounters. Additionally, the way we began to walk upright had the effect of moving human female genitalia more to the front which enabled her to be more selective about her partners. With the parental instinct being shared among both males and females combined with the fact both parties also are able to derive a great deal of pleasure from the experience, both sexes end up being more closely aligned with the goals of a sexual encounter vs those of our most recent, more ape-like ancestors.

To bring it back around, I understand what you are trying to say here, but disagree with your conclusion. Evolution selected modern humans to engage in sexual encounters that will not always result in a pregnancy, unlike eating which your body will always attempt to process the same as the last thing you are.