I agree that there is a real risk of introducing bacteria from the rectum into the vagina, which is why it is okay to go from pussy to ass but not the other way around without taking precautions. It's worth mentioning that women and girls should also wipe their anus in the direction away from the vagina for the same reason.
I have rules I follow so that I do not put my wife at risk of an infection, as every few days she likes to be fucked anally. She loves me teasing her beautiful anus with fingertips, generally playing with it and when she requests me to, deeply inserting a finger (or two!) ...
1) Fingers of my right hand are reserved for pussy focused foreplay. They must not touch her anus at any time.
2) Fingers of my left hand are primarily for her anus. They MAY be for her pussy IF AND ONLY IF they have NOT already touched her anus.
3) My uncovered penis may switch from unprotected vaginal intercourse to unprotected anal intercourse but then MUST NOT so much as nuzzle her vagina, never mind re-enter it. A condom must be used if switching from anal intercourse to vaginal.
3a) Alternatively, a condom must be used for anal intercourse if there will be a switch to unprotected vaginal intercourse.
In practice, we use condoms only very rarely, and as my wife feels less satisfied if I have not ejaculated inside her body. She usually wants me to ejaculate deep inside her vagina, and she likes to feel my semen trickling out afterwards. If I have screwed her ass then she will want me to ejaculate inside her rectum. She then enjoys the feeling of wetness between her buttocks as my semen leaks from her anus.
My final point is that unprotected anal intercourse is always quickly followed by us both going to the bathroom together, where we both watch each other having a piss, and my penis is given a thorough but loving wash by my wife.
So we both say – enjoy anal sex but set sensible rules for penile and vaginal hygiene.