Do "we" tend to consider a bare butt obscene?
Without other factors, no. Here is a photo on a page at webmd of a gal in thong swimwear.
One funny thing is that 50 to 60% of the viewers of the Victoria's Secret Fashion Show are women. Jack ents for CBS. Of Victoria's Secret he says he has his own decency filter: "Do my two teenage daughters want to see it? Will my wife watch it? And will my mother not turn it off? And this show gets through that filter."