There is no doubt that anal sex can be risky -very risky. Even when you use plenty of lubrication during anal sex, the tissue inside the anus is much more delicate than your skin and can tear. Your skin has many layers of dead cells that act as a protective barrier. However the tissue in the anus has no such protection and can tear easily.
The anus is usually held closed by a sphincter, which is a muscle that goes around the opening of the anus. Here the person who is going to receive anal sex (the bottom) is shown lying face down, and the penis is shown at the right before it comes in contact with the anus.
The penis begins to put pressure on the anus as the man tries to push his penis into the anus. The extra pressure on the anus causes the sphincter muscle around the anus to contract tightly. This sphincter tightening is a reflex and it happens automatically, as the body was designed to keep things from entering the anus from outside.
The penis has begun to push deeper into the anus. The tight fit of the penis in the anus (which is closed by the anal sphincter) causes small tears in the interior parts of the anus even though there may not be any obvious signs of bleeding. Slow penetration with lots of lubrication can reduce this risk but not eliminate it.
These tears while very tiny are much much larger than the size of a virus. In effect they act like a super-highway providing fast and easy access for viruses into the body. For this reason, anal sex is the riskiest form of sexual activity when it comes to the transmission of HIV/AIDS. Anal tears provide an opening for all the other STDs as well. It may be possible for repetitive anal sex to lead to weakening of the anal sphincter, which is the muscle that tightens after we defecate. Once weakened, feces can escape the anus against our will -not a very pleasant situation, but also not that common. This is why you should always use a condom and plenty of the proper lubrication when engaging in anal sex.