I'm sure that this was meant as a joke, but actually it's a good question. After you have a bowel movement, how big in diameter is the fecal material in the toilet? Years ago I had a gay friend who was in exactly the same situation as you. He wanted to be fucked, but his ass was just too tight. He remarked that the material in the toilet was about the diameter of a candle, much smaller than the vast majority of dicks.

Your remark that dicks, ears and just about every other body part comes in a variety of sizes, so why not assholes is spot on. I've put my dick in plenty of them and some were so tight it was like fucking a vice, while others were so loose it was like fucking an open window.