callipygian: adjective cal·li·pyg·ian \ˌka-lə-ˈpi-j(ē-)ən\ having beautifully shaped buttocks.
From Ancient Greek καλλίπυγος (kallípugos), from καλλι- (kalli-, “beautiful”) + πυγή (pugḗ, “buttocks”).
William Etty (1787-1849) is probably the most controversial artists of whom you have probably never heard. A high-minded bachelor whose private life has defied all attempts to unearth smut, Etty was acclaimed in his day but eventually sidelined because of his defiance of moralizing, often hypocritical, critics. He was a shy man and remained a bachelor all his life, which at the time was practically a statement. There is no way to confirm Etty's sexual orientation since he's long dead and lived in a time when no one really identified as gay. However, the paintings may speak for themselves. He was a successful Royal Academy artist, but his work fell out of favor after his death. But while he was an active painter he was both admired and condemned for his detailed renderings of the naked human body, often focusing on the buttocks.
Critics felt he focused too much on the female buttocks, but if you Google Image search for his work, you find a surprisingly large number of male nudes, many with a focus on the male buttocks as well. Seems none of his contemporaries were interested in commenting on that, but it's obvious that Etty's was a butt man, no matter his orientation.
Whereas his contemporaries, like J.M.W. Turner changed how people saw art, Etty wanted to change what people saw. Etty broke the rules of decorum by painting humanly realistic nudes rather than idealized gods and goddesses. Most of the criticism questioned the appropriateness of Etty’s female nudes, while the male nudes quite often found praise as “heroic.” Tragically, the critics got personal in their comments, essentially charging Etty with deliberately trying to corrupt the viewing public.
“He is a laborious draughtsman, and a beautiful colourist,” one critic began innocently enough, “but he [Etty] has not taste or chastity of mind enough to venture on the naked truth […] we fear that Mr. E will never turn from his wicked ways, and make himself fit for decent company.” “[T]he spectator can see in [Etty’s female nudes] nothing beyond the portrait of some poor girl who was necessitated to sacrifice the feelings of her sex for bread,” another critic accused. “Nudity is all that the artist has to show us, and when unassociated with anything like incident or sentiment, the spectacle is offensive.” Etty defended himself as an innocent lover of nature’s greatest creation—the human form. Even after evoking the Biblical phrase that “to the pure of heart all things are pure,” Etty’s explanations fell on deaf ears.