Oh no, it can totally be trained. If you're doing it properly then that's how it's supposed to be done.
There's just two thing people don't seem to think about.
#1. It DOES need to be trained. Far too many just plunge on in, and that's why there is a frighteningly large market for prosthetic sphincters, for those who ruin theirs playing porn star.
#2. It is NOT as elastic as a pussy. The vagina is MADE to stretch for child birth. While the rectum can be trained, physiologically it is NOT the same, and can not be expected to be… which again leads to the problems of #1.
For me it's those two issues… as well as a hygienic issue. There is a reason that area is basically separate from the rest of the body.