By that logic then it's no go with the mouth too. That also being a, "one way chute." The small percentage of the time it's not, I don't want to think about.
The anus is an erogenous zone. It's loaded with extremely sensitive nerves designed to receive pleasure. It also is right beside the prostate, an organ also capable of receiving intensely pleasurable stimulation. So long as one's hygiene is good and there's no immediate need to defecate, the anus is pretty damn clean. It does not hold fecal matter save for immediately before and during defecation.
The psychological effects, for many men, of penetrating the anus of another person are remarkably and wonderfully intense to say nothing of the physical pleasure it provides to the penis.