The natural protection which the vagina provides to the virus is broken; then women with lesions can become infected, as well as men.
Damage to the vaginal mucosa can occur in other ways too. For example, a yeast infection or just aggressive intercourse would increase the risk of contracting HIV from a HIV+ man. Heterosexual intercourse may not spread HIV as efficiently as homosexual intercourse but it still spreads it.
Also a man is still at significant risk if he has unprotected sex with a HIV+ woman who does not have a damaged vaginal mucosa. This is because the vaginal fluid contains a high percentage of CD4 T cells, which are the hosts for the virus. HIV can also sometimes pass through the glans penis mucosal membrane.
STDs are much higher in Third world countries, as well as in Eastern Europe.
That is the reason for the spread of AIDs there, among non-drug-using heterosexuals.
An STD infection would certainly increase the transmission efficacy of HIV. But having unprotected sex with a HIV+ person who doesn't have any other STDs is still very dangerous.
...the expected breakout into the heterosexual community has not happened for the reasons above.
Don't you think people have also changed their behaviour? Condoms are used much more frequently in the USA and Western Europe than they are in Eastern Europe and the third world. The proper use of condoms is the only way to prevent the spread of AIDS, apart from abstaining from sex altogether.
That's bullshit. How about two people in love get married and have a monogamous relationship? The problem is that certain people, particularly the activist homosexuals, can't fathom the thought of not having 20 or more partners every year.
Now, you don what you want, that doesn't bother me. But don't tell me that I am spreading some epidemic if I have sex without a condom. Neither myself nor my wife has AIDS. We love each other and are in a monogamous relationship. We have sex without condoms. We are not contributing to the spread of AIDS. End of story.