Q:

A:

This is a complicated question to answer as there are many people for whom anal intercourse is a regular and enjoyed sexual activity.  But it is medically risky behavior nevertheless, even if condoms are used as a barrier for STDs.

The anal sphincter muscle is not anatomically designed to comfortably admit external objects--it is designed to relax and stretch when stimulated internally by rectal fullness from stool.  The automatic reflex is for it to contract and tighten when pressure is applied externally.  So relaxation of the sphincter for external penetration is learned over time because otherwise it is very uncomfortable, and must only be done with gentle continual pressure, and lots and lots of lubricant. The risks, even with gentle insertion, are laceration of the anal tissue, and rectal mucosa, resulting in pain, bleeding, and difficulty passing stool comfortably. 

Any presence of blood can potentially expose the insertive partner to bloodborne STDs like Hep. B, Hep. C, and HIV.  In addition, exposure to stool can result in urethral infections in a male insertive partner.

The receptive partner is at more risk for contracting STDs if there is trauma (even microscopic) to the anus or rectum due to the potential presence of virus in semen, if ejaculation takes place in the rectum.  Human papilloma virus also is likely to be spread anally due to this trauma to the anal and rectal tissue, and some of the most difficult persistent HPV infections we see are chronic anal warts, both external and internal to the anal sphincter and they are exceptionally difficult to treat, often requiring surgery to remove.

Aside from the traumatic and infectious risks, there is the risk of sphincter tone (tightness) loss over time due to repeated dilation for insertive intercourse.  Many receptive partners experience stool incontinence (leaking of stool or poor control) when they have anal sphincter tone decrease.  This, needless to say, is very bothersome and uncomfortable and has to be surgically corrected if it becomes chronic.

Lastly, there is increased risk of spreading gastrointestinal pathogens through anal contact--whether it is bacterial infections like salmonella or E. Coli, or parasitic infections like Giardia.

The bottom line (pun not intended) is that having anal sex is a form of sexual expression enjoyed by some people, and that involves potential discomfort and risk to both partners. 

                     ~The Doc