Yep — pregnancy. In order for someone to become pregnant, sperm needs to make it into the vagina (and then through the cervix and into the uterus, etc.), and that could be possible (though unlikely) depending on what position you’re in, she explains. So ejaculating in someone’s butt probably isn’t the best idea if you're not also using another form of birth control.
I personally have just started experimenting with anal sex and from my experience, it sounds worse of a painful experience than it really is. My misconception about it, that I think a lot of other girls have too, is that the initial pain of… insertion… lasts through the entire insertion process, or in more blunt terms, that the pain you feel when a guy first inserts his penis “head” in lasts throughout the entire time he continues to push inward. This was not the case at all, for me at least. I’m not sure if that is what the term “rimming” is, but once my partner and I got past that initial step (which did take some practice and patience, as described in the article), it was smooth sailing.
Christian texts may sometimes euphemistically refer to anal sex as the peccatum contra naturam (the sin against nature, after Thomas Aquinas) or Sodomitica luxuria (sodomitical lusts, in one of Charlemagne's ordinances), or peccatum illud horribile, inter christianos non nominandum (that horrible sin that among Christians is not to be named).