Hi atomXY
I would say that obviously the bottom is not designed for penetration and there are risks associated with that kind of activity. However I would imagine that most people would concede to that point.
I am slightly confused as to why this is important though. Humans do all manner of things that they weren't designed for and that can cause them ill health but they still do it, often in the persuit of pleasure.
Sexual intercourse is a dirty risky business no matter what way you do it, some practices are more risky than others. Anal sex isn't just for same sex couples either.
I haven't read your links but will try to have a read today.