In some episodes, Benson wears outfits.
Even though Benson is usually not seen wearing clothing, in Fortune Cookie, when his robe flies off of him, he is shown with a visual rear end. This also happens in Brain Eraser, except his hatch unlatches and doesn't show his rear end. This may indicate he wears some kind of clear outfit over him, which covers his nudity.
In Skips Strikes, he wears a red and white striped bowling shirt, without any pants. In some occasions, at his apartment or even at Pops' house, he is shown wearing a white bath robe. He also wore a red shirt with flowers in Meat Your Maker. And in See You There, he wore a costume of El Zoro