It has a lot to do with how the figures are being painted. I don't think every nude painted by a man is necessarily sexual, but it does seem to be the majority of them that don't really have any point except to look at attractive naked women, and the artistic merits of pieces like that are pretty questionable even when the technique is good.

As for why to bother debating it beyond the model and the painter, that's because they're not the only one involved. If the model and painter got together to do the painting and then the artist immediately threw his work into a fire, or stuck it in the basement to rot, then sure. But what people are talking about is gallery space, pricing, and ultimately how much importance we want to give them in our cultural fabric. People like art because it evokes certain thoughts and emotions in the viewer. A painting of a perfect looking woman who is apparently nude except that her feet are naturally formed into a high-heel shape (Allen Jones, Backdrop) really does nothing for me, or I reckon for most other people. It literally looks like cartoon porn with a thin veneer of "art" put on top for plausible deniability. Checking out his other works certainly didn't change my mind. Personally I would have no interest in visiting or supporting a museum that thought this was good art. It's equivalent to a schoolboy drawing titties on his desk, except that this guy has developed a better technique. Unless it's jerkoff material, what's the point? And if it is jerkoff material, why would anyone care about it as art?