These cells have a layer of mucus that, along with other secretions and the thicker, more flexible vaginal wall, protects against abrasion and infection. The rectal wall has no surrounding muscular support, and it secretes a small amount of mucus that does not protect well against abrasion. But the key difference between the vagina and the rectum are the cell types and the thickness of the cell layers. The two orifices may feel very much alike to the intruding finger or penis. But one orifice is prone to repel, the other to admit, whatever microorganisms come along for the ride. . . .
Anal intercourse stretches the opening to the size required for a large bowel movement.
The problem, however, is not the size of the opening but the direction and repetition of the movement. The anus is a one-way valve, stimulated to open only by pressure from inside, and stimulated to contract by pressure from outside. Sudden or inadequately lubricated penetration can tear the anus itself. But more commonly the cumulative effect of anal intercourse is to cause dysfunction of the anal sphincter muscle, and the result is chronic incontinence or urgency of defecation for about one in three men who regularly engage in the practice. Nor is that all.