(The nude-but-not-sexual viewpoint is pretty valid, but some of the fine art is definitely sexual once you know the context, and some of it was controversial in its time e.g. https://en.wikipedia.org/wiki/Le_D%C3%A9jeuner_sur_l%27herbe )
Source? To my reading, this is a decisively late Christian/Puritan perspective on sex and the human body.
Of course, exactly what counts as nude (or public lovemaking) and just how draconian the repercussions for transgression are, varies greatly.
Some snails mate while hanging down from a tree branch by a thread of slime. That's about as private as a snail can get.