A condom is a mechanical barrier that prevents transmitting many sexually transmitted diseases, including AIDS. Not using a condom significantly increases the chances for transmission. The chances for transmitting HIV infection is 50 per 10000 exposures to an infected source in receptive anal intercourse (from you to your girlfriend) and 6.5 per 10000 exposures to an infected source in insertive anal intercourse (from your girlfriend to you).


It takes even small ruptures in the anal or penile mucosa for HIV to be transmitted. Those ruptures might be so small that they cannot be noticed with the eye. The semen also contains HIV particles and can transmit HIV when it is ejaculated in the anus.


As for the white fluid coming out of the anus during anal intercourse, it cannot be determined for sure what it is without an additional examination. It might be just mucus produced by the anal mucosal glands, or it might be a sign of an infection. Either way, you are both advised to visit a specialist for infective diseases to check for a sexually transmitted disease.