As a nurse, I've read this thread with interest, and a smile most of the time. First, as with any muscle, the sphincter can be stretched if done gently without causing any long term lasting effects... as long as it is not given the chance to atrophe (do those keigel excercises to keep up the elasticity!) and given a chance to return to normal and not remain in a stretched condition for extended periods of time without a rest. Second, as long as you KNOW without a shadow of a doubt that you are in a relationship where there is no chance of death by sex and both partners have tested negative for chlamydia, human papilloma virus, trichomonas, syphilis, gonorrhea, gardnerella, and herpes then a condom is not necessary as long as 1) there is no switching from anal to vaginal sex without cleaning up first and 2) the male urinates right after to flush the bacteria from the urethra and washes well to eleminate them from crawling right back into the warm wet environment they like so well.

To make anal sex play enjoyable and healthy, keep these tips in mind: Because no lubrication is produced by the anus, a lubricant should definitely be used to help the insertion of anything -- finger, penis or sex toy. Water-based lubricants are fine. Some of these can be purchased at drugstores or at a store that carries sensual accessories. Other more greasy lubricants are preferred by some who intend to spend a longer time at anal play. If you are using a latex based condom, remember that petrolium based products break down latex, exposing you to the risk of a STD.

For additional information on this topic, I recommend Anal Pleasure and Health: A Guide for Men and Women by Jack Morin.

~steps down from the lecture podium with a sheepish grin~