1. The female art model is not naked, she’s nude, and not even that: she’s covering her sex (that’s called “implied nude”). Of course she’s depicted as such, she’s representing all the artist’s models — she’s a symbol, not a named individual. I’m very sorry but since the dawn of art, female nudes have been the subject of art. And in this painting, there’s absolutely nothing sexist to be seen. When a painter needs to represent an art model, he’s not going to be able to do so when he depicts the model clothed.
2. Are the countless images of a naked Christ on the cross (in this case naked and not nude — he’s not supposed to be an art model…) also depictions of the “normalcy of modern sexism”? Is Michelangelo’s David (and countless other male nudes from the Renaissance) sexist if we presume that the artist was homosexual?
3. I’d love to read more articles about the *organised*, *structural*, and *institutionalised* sexism in all male monotheistic belief systems. It would help the case you’re trying (but failing) to make, in a far more effective way. Why? Because contrary to this painting, not even made in recent years (“modern” sexism?), those belief systems still have a devastating effect on the image of “woman” every single day, since 2 millennia.

Your “opinion” about the (semi-)nude model being some kind of “decorative element” in the painting serving the egoism/narcissism of the artist, is totally biased, personal, and not correct. You really should study some history of art before making such claims — if the artist would still be alive, he’d have a few things to tell you… The artist’s model is seen in the centre of the image, standing next to the artist — she could be his wife or partner, and if not, she’s at least someone dear and important to him, not just a decorative element serving his male sexist pride. That she’s a figure of importance can be “read” from her prominent place in the centre.

And finally, to compare this painting to a selfie of today, is misunderstanding the motives behind the making of such paintings in Courbet’s time versus the ones behind clicking a button on a phone to uplift one’s self-esteem. To say the painting is depicting “the normalcy of modern sexism” (versus women) is as correct as claiming that all crucifixes featuring a naked Christ were created to make him into a male sex symbol.