Anal sex is more likely to cause tears in the epithelial barrier of the rectum which significantly increases the risk of HIV transmission.

Really puts a spin on that whole African Aids issue prior to Internet porn existing.

Add more people into it ,because they are aware of it (And how popular it seems), and all of a sudden "Condoms prevent pregnancy so let's go natural" now opens up a new vector for HIV infection in those who aren't male and homosexual.

As for the trans thing, I don't know - but I'd say that there is some correlation with causation too.