The fact that you can't get pregnant during anal sex does not negate the need to use protection. Unprotected anal intercourse is not safe for a variety of reasons. Here are four reasons to use condoms for anal sex.
Reason 1. STIs can be transmitted during anal sex. Sexually transmitted infections can be spread by any kind of sex (vaginal intercourse, oral sex, anal sex). In fact, anal sex carries greater risk for passing STIs as the anus is more prone to microtearing than the vagina. If the tissue inside the anus is torn during anal sex, bacteria and viruses enter the bloodstream more easily. Infections that can be transmitted by anal sex include, but are not limited to, HIV, gonorrhea, herpes, hepatitis, and chlamydia.
Reason 2. Unprotected anal sex can cause urinary tract infections. The anus contains more bacteria than the vagina. These bacteria can get inside the male partner's urethra and cause UTIs such as cystitis. Women often get urinary tract and vaginal infections during anal sex when they switch between anal and vaginal sex. That is why you should not only use a condom during anal sex, but also change it when you switch to vaginal intercourse.
Reason 3. There is a small chance that you might get pregnant. You cannot get pregnant during actual anal intercourse, but if semen gets to the perineum (the area between the vagina and the anus), it can leak into the vagina. You can prevent unplanned pregnancy by using another form of birth control (i.e. pills) or asking your partner to ejaculate elsewhere – such as on your back or in a tissue – but the risk of STI transmission still remains.
Reason 4. Receptive anal sex without condom can cause diarrhea. When semen gets into the colon, it can cause diarrhea. It is the body's way of getting rid of it. Once again, you can ask your partner to ejaculate somewhere to avoid it or go to the bathroom immediately after the intercourse and clean up, but don't forget about STIs.
The bottom line is, condoms make anal sex safer, more hygienic, and less messy. We strongly recommend that you always use condoms for anal sex (and don't forget to change the condom every time you switch between different kinds of sex). You should also keep in mind that oil-based lubricants must not be used with condoms because they damage latex. Water-based lubricants are the best choice for anal sex. Be sure to use enough lube (we mean lots of lube!) to prevent pain and damage to the anus.