In fact the germs (bacteria) in the anal cavity are so inhospitable that sperm ejaculated in to your anal cavity would most likely die before leaking out of you. Even them it would have to flow into your vagina then swim up and make contact with an egg. This is very unlikely to happen.
If you are to continue to have anal sex make your partner use a condom for if he gets an infection he can pass it to you. If you get an anal cavity infection it will be very uncomfortable and be considered and STD.
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