The anus can relax and accept a penis only when lubricant is added and because it already is supposed to relax to allow waste out. I'm sure most of us have taken craps larger than a lot of cocks out there, so obviously the anus can relax enough to let a penis in if it throws larger shit out.

The prostate stimulation being good or better than anything doesn't exactly prove much either. Humans are no stranger to finding ways to abuse features of our body to get megadoses of the happy chemicals. Drugs? Concentrated sugary foods? All of these are much more pleasureable than anything in nature because they give more stimulation to parts of our brain than anything that naturally exists. If we evolved to like sugar because fruits are healthy, then man-made sugary foods will stimulate that on overdrive. If the prostate evolved to make vaginal sex pleasureable so that people reproduce, then stimulating it directly would give stronger feelings of that pleasure.

And if you take the pleasure out of sex, it is very much enduring. Especially for inexperience bottoms, sex can be a bit of a chore, from cleaning to making sure you aren't so tight that it hurts. A large, unlubed cock and a tight hole are a recipe for pain without much pleasure for a new bottom, so please don't disregard the pain some men go through in search of pleasure