In this version of the future, people hang sexually explicit art on their walls in the same casual way you might hang a still life or inspirational cat poster on your walls today. Alex has artwork like this in his bedroom—if dude had a computer, he would feel no need to use Incognito Mode. In the society of A Clockwork Orange, there's no need to keep sexy imagery under wraps.
The artwork adds enormously to the sexually charged atmosphere of the film…and also to the misogynist atmosphere of the film. The only depictions of men are in graffiti, which is done for comic effect, like a cartoon penis scrawled in a bathroom stall. All of the serious artwork is of women, which perpetuates the belief that women are sex objects and meant to be used by men.
Alex even taunts the catlady for her giant phallus statue, which she calls "a very important work of art." It's the only bit of erotic artwork in the film that is male-oriented and Alex kills her with it. In this world, (straight) women aren't supposed to take control of their own sexuality.