(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.
This is true only for very unusual definitions of “always” and/or “fine art”; history is more than Victorian England.
“No urns,” he said at last.
“What urns?” said Nobby.
“Nude women are only Art if there’s an urn in it,” said Fred Colon. This sounded a bit weak even to him, so he added: “Or a plinth. Best is both, o’course. It’s a secret sign, see, that they put in to say that it’s Art and okay to look at.”
“What about a potted plant?”
“That’s okay if it’s in an urn.”
— Thud (2005), Terry Pratchett
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.
Because otherwise art is impossible.
If you are a painter, and you need to draw people, you learn to draw naked people because you need to learn how human anatomy works, where various muscles and wrinkles are. In your careers you won't be just asked to draw dudes in T-shirts, you will have to draw or animate people in various clothes and state of dress or undress, torn clothing, gladiator games, etc.
There is great deal of knowledge that they actually have to learn, how to draw realistic deltoids in different body positions, or under stress to show that a person is putting in a great effort to support a great weight, or is in a fight. A drawing by a great artist should make a medical professional happy.
That's why they hire nude models to stand around in various posses, and they aren't all pretty, some of them are old people because they have to learn how to draw wrinkles and old skin.