Ahh, I think I misunderstood what you were getting at. To address your original question

But I'm curious to the reason why that is.

There's an innate repulsion towards it. There must be some evolutionary explanation for that.

Seems pretty unlikely to me personally, we don't see prudishness towards natural bodies or their functions in the animal kingdom anywhere but humans afaik. This seems anything but natural to me. I think even early civilized people realized this, as the Garden of Eden has Adam and Eve naked in their natural state and unashamed. There are examples of primitive peoples who do not have our aversion to the natural body or its functions.

Or even a social one.

Yeah I think this must be something that evolved socially. My best off the top of the head explanation:

Perhaps this evolved when humans started living in groups that were larger than Dunbar's number. I.e when our relationships with our neighbors became less intimate and full of strangers, and we perceived a greater sense of threat to our person from exposing of intimate parts, or even becoming vulnerable to danger while taking a crap or having sex. Or that the strangers would not respect the social boundaries with the greater anonymity afforded more dense living arrangements. People can't police themselves at greater than Dunbars number for this reason, and need dedicated law enforcement with prescribed codes of conduct. That would be my theory. When social arrrangements became 'unnatural' i.e. transcended the arrangements we evolved for and practiced for eons, we started to exhibit 'unnatural' behavior which was necessary to adapt to our new artificial social environment, as this new environment needs to be more orderly than the small band in oreder to function with its greater complexity and more moving pieces