Surely it's in the mind of the reader what they find erotic and this one small minority here finds anus very erotic so so so arseholesssssss
In terms of 'anus', I think it's better kept admired than spoken and written of. Maybe in a super veiled reference we can refer to it, but bringing that little hole out into the open doesn't push my buttons. We talked about 'penis' and 'vagina' before, it's the same clinical dilemma. 'Anus' is the sound a clinician makes when they sneeze.
Just to get a little pervy(along with everyone else, it seems) I adore the erotic quality of my soon-to-be wedded wife's anus and someday hope to write a quality poem about said adoration.