You're all wrong, when she's in certain positions (like doggy or reverse cowboy) and very wet, the D can go anywhere. And boy! Does it go in terribly easy.

For one thing, those positions make the D point down, when normally erected it will point up. For another, if it's fully lubricated and both are also sweating it will slide in like a fish in your hands. It has nothing to do with how big the anus is. In this case is her entire body weight pushing on a D.

It's bad because you never want that to happen and it hurts like hell but it's an accident and I guess that you may not even notice if you're not watching, maybe just enjoying the moment.