The skin around the anus is thin and not stretchy. So no matter how much lube used, a virgin ass can tear..... considering the size of the penis.
The anus bleeding a bit from tears in the skin would be an effect of anal sex. If it's more than some spotting on a tissue, there is cause for concern.
The location of those tears are subject to infection so it is important to have a cleaned colon and most important, condoms.
Anal is definitely something to prepare for. Ideally an empty stomach, clean colon, relaxing in a bath, and a butt plug for a little while before hand to stretch it a bit. It is a muscle you know!
I find it to be an intimate thing I designate for a close lover.
After all, gotta have something not everyone's had a piece of! LOL!
More power to the girls with ass of steal though. My ass wishes it was as inviting as yours.
Also, this might help. If you look past the moral judgement it can be informative.
Anal Sex: A Dangerous Trend