Sorry if I only reply now, I was traveling. Also, I can't think of a useful comment. I just see it very differently, and the reason I ask "why has it to be categorized as sexist" is that I worry about the implications. For example if it is assumed that it is demeaning for a nude picture of a woman to be posted, but not if it was a nude picture of a man, the implication seems to be women should somehow be ashamed of the way they look nude.

I probably can't describe it very well, but I sense lots of weird implications of calling it "sexist" which I think are not really what the people calling the sexism flag had in mind.