I say not everyone is made for receptive anal sex. Some people can do some coke can shits, but not everyone. When I was in high school I worked cleaning the elementary school. One day me and the lady I worked with found a shit in the elementary school bathroom that had to be broken up to be flushed. I don't think an adult did it. There were staff bathrooms. These were little tiny kid height toilets.
In porn you can see some people have little difficulty even with sizable members. And some enjoy it very much. But I say there is anatomical variation. I suppose the sphincter can be stretched over time, and one can search for more compatible partners, but this may not be for everyone.
I like a finger in my ass - a lot - prostate massage. But a dick is pretty large for me to insert. I've done it before with a lot of lube, but caused some minor bleeding and was as painful as pleasurable. | 
