the anus did not evolve to have objects enter it so when a penis or a dildo or anything else enters the hole, hundreds of microscopic fissures are created that release blood and enable pathogens to flow from the anus up the penis. oral sex doesn't have any additional hygienic risks as it also evolved for objects to enter it, as well as to crush them, so it is actually even more efficient than fighting off pathogens than the vagina.
the anus is near the vagina, so this is how the verse in the bible (and probably referenced in the quran) when it says "a man should not lay with a man as one lays with a woman", that is, having sex with him in his anus.
zirkza: did that town happen to be Sodom?