You might want to read up on the cultures of some of the African tribes with the highest proportion of HIV in the straight population. The majority of the straight victims are women, who are also victims of certain cultural practices which involve homosexual behavior.
In either case, it is recreational sex with multiple sex partners where at some point at lease ONE person in the mix had homosexual sex.
Most of southern Africa is living in the 21st century and references to tribal practices aren't as relevant as trade and work patterns that take men far from home for long periods of time to places where prostitution and drug use are common. I read a profile of a village in Swaziland where many people caught it not only from their husbands, but from close relatives either in the womb or in caring for the bleeding sores of the sick. Hygiene is abysmal.
Most Southern Africans are CHRISTIANS who completely reject homosexuality in every form--moreso than many Christians in America do. That's why the Church of England in Africa has rejected the Episcopalians here.