If you want the dirty details why, just look up a photo for “Balanitis”...
ironically, in South China and Iran, HIV is spread via IV drug injection, and in Eastern Europe/Russia via blood transfusions and dirty needles.
It is thought that French speaking Haitians ho had worked in the Congo brought it back to Haiti, and the sex tourism spread it from there. Cuba did not have the problem, because their soldiers in Angola who tested positive were quarantined for awhile.
HIV was also spread, for a while, through contaminated medical blood products. Arthur Ashe seems to have contracted it that way, getting a transfusion when he had heart surgury. There were several cases of hemophiliacs getting HIV.
That has been cut back since the smart guys developed tests that would detect fairly low levels of HIV positive antibodies in blood. Still, it is a good idea to stockpile your own blood before you have scheduled surgery.
I play racquetball with a urologist, and I asked him about this - why does Africa have such a higher rate of AIDS with women. He mentioned the circumcision effect. In fact, he spent a few months in Africa setting up circuncision education centers.