There's more to it than just STD free
As well as the very real chance of catching something you do not want, this is how it was spelled out to me by a doctor. Sperm has a complex chemical composition. By its design, each sperm is a foreign object that is coated with a protein that supresses the immunological responses of the recipient. Without this property, which apparently is passed onto a fetus, the recipient's body would treat it the sperm as an invasion of foreign objects (which it is) and an immunological reaction would ensue. This suppressive property that each sperm carries ensures that the sperm can make the passage to the egg and, also, that the foetus is not attacked by the mother's body as a foreign object.
The vagina will, by design, drain off excess sperm and channel it, thereby protecting the recipient's body from its presence. The rectum, by design, will not. While the thick walls of the vagina can handle the invasion well, the thin walls of the rectum cannot. Also, because sperm tends to hang about in there, the full effects of the immunological suppression take effect, along with a host of other reactions due to the volatile chemical composition of the sperm.
This in turn, along with the damage done by fucking a hole that was designed for exit only, creates in the recipient a cascading series of reactions on a cellular and hormonal level which can, apparently, cause some serious imbalances in the recipient's body (with repeated exposure, the water balance in the intestinal track gets off kilter resulting in diarrhea for instance).
Also, apparently, the rectum does not register pain and so damage can ensue in the course of the ass fucking, and the recipient would be none the wiser. But that damage can take the form of fissures in the rectum that would then release a bootyful mix of fecal bacteria and sperm into the bloodstream, which would in turn release a freaky assortment of measures in defense of that.
Does it still sound appealing...? Sure looks cool in the moovies
