The bacteria in the anus is such that it can cause a serious infection to your boyfriend. He could get a urinary infection or even a kidney infection should any of the bacteria that live is in the anus makes its way into his penis. It is therefore recommended that he use a condom when you and he engage in anal sex. Bedfore you switch to vaginal or oral sex his penis and groin area should be washed with hot soapy water to insure any germs are washed away. [ adviceman49's advice column | Ask adviceman49 A Question ]
Dragonflymagic answered Sunday February 19 2017, 9:28 pm: The anal cavity and the vagina do not have any connections inside you. So any semen that goes in back will not make you pregnant.
However, depending on how you are positioned, a good amount of his cum can leak back out of there just as it can from vagina. Once it has leaked back out, if it instantly runs toward your pussy lips if they are really wet, the sperm may be able to travel into the vagina. This is a very slight chance.
I believe the best practice is to use the bathroom right after and strain to see if anything leaks out. Then wipe only in back not dragging it forward to vagina. Also, an extra tip for after sex in vagina, it is a good idea to try to pee to clear out any possible germs that may have gotten inside your vagina. [ Dragonflymagic's advice column | Ask Dragonflymagic A Question ]
Attention: NOTHING on this site may be reproduced in any fashion whatsoever without explicit consent (in writing) of the owner of said material, unless otherwise stated on the page where the content originated. Search engines are free to index and cache our content. Users who post their account names or personal information in their questions have no expectation of privacy beyond that point for anything they disclose. Questions are otherwise considered anonymous to the general public.