While the bacteria residing in the vagina are meant to protect women, this is not the case in the colon. Those are meant to digest food. In other words, if you have sex with a healthy girl, her vagina is harmless, but if you just fuck her ass, both of you can get sick. To prevent infection, all you need to do is empty the colon.
There are two simple steps to do it, and both of them should be done by the girl anytime before the anal action. She either should not eat for several hours, so that no food gets inside from the top. Or the other solution is a simple colon cleansing enema, which means that she should wash her inside with about half a liter (2-3 cups) or warm, but not hot water. This is the safest method, since it cleans out most of the bacteria, as well. Once it has been done – and the man does not even need to know about what is going on in the bathroom – the time for anal sex has come.
One more advice should be taken for the woman's health: once your cock was in her ass, resist the temptation and do not follow the examples seen in porn, so do not put your dick back into her pussy, because there is a high risk, that she can get sick, while you will not. If it happened, then go to the drugstore and get something to clean her pussy (e.g. Iodine) as soon as possible. And just in case, you can use a skin disinfectant on your cock, as well.
It might sound too complicated, but if you love anal sex, these simple steps should be followed, because ass is not only tighter, it is also more dangerous than the pussy.