“All art is immoral.”

Albert Edward Elsen (1985). The Gates of Hell by Auguste Rodin. p. 131
1950s-1990s
Context: In art, immorality cannot exist. Art is always sacred even when it takes for a subject the worst excesses of desire; since it has in view only the sincerity of observation, it cannot debase itself. A true work of art is always noble, even when it translates the stirrings of the brute, for at that moment, the artist who has produced it had as his only objective, the most conscientious rendering possible of the impression he has felt.

Ils faisaient à autrui ce qu'ils ne voulaient pas qu'on leur fît, principe immoral sur lequel repose tout l’art de la guerre.
Tr. Walter James Miller (1978)
Variant: They did unto others what they would not have others do unto them, an immoral principle that is the basic premise of the art of war.
Source: From the Earth to the Moon (1865), Ch. X: One Enemy v. Twenty-five Millions of Friends (Charles Scribner's Sons "Uniform Edition", 1890, p. 50)

Letters from the Earth (1909)

from New Music, Outmoded Music, Style and Idea (1946); as quoted in Style and Idea (1985), p. 124
1940s

Albert Edward Elsen (1985). The Gates of Hell by Auguste Rodin. p. 131
1930s and later

The Soul of Man Under Socialism (1891)

Daniel Martin (1977)