I think you make an excellent point. There a millions of heterosexual couples enjoying anal sex and anal play (I can see Celcipriss cringing at the very thought - or what passes for thought) without developing AIDS. Sex Film workers (gay and straight) also engage is inordinate amounts of anal sex and again without developing AIDS.
The hundreds (thousands?) of nurses/doctors who have contracted the disease from infected sharps tells us that it is not toxicity or diet or as one homophobe put it "fudgepacking" that causes AIDS but the HIV.
You would be publicly ridiculed and rightly so, to suggest that all the 25 Million people in Africa living and dying of AIDS got it from gay anal sex.
Many of the people who got it from tainted blood never had anal sex or any kind of sex. Yet they were infected. This includes many children, and others that did not practice any of the behaviors described.
Finally you don't even need to understand biology all that well to see that the method of infection is not due to the some of the proposed alternatives. The calculations can get complicated but it can be proven strictly with mathematics that the method of transmission is due to an infectious agent that is passed through blood borne contact. Though, that would be an intellectual exercise since it has been shown clinically.