Mostly hetero transmission, and sure, high promiscuity. There are SO many factors (and I'm no expert on this, so just take this as a quick spin):
* migrant labour systems -- leaving the man to go to the city for work, and the wife/children at home in the rural areas -- are still largely intact (this was the basis for 'apartheid-capitalism' but continues unabated because of what a generation of marxists here called 'articulations of modes of production');
* desperation-induced prostitution at truck-stops on major transport routes, and in SA's several major red light districts;
* massive problems of rape and domestic violence;
* ghastly US-style lifestyle marketing, which includes also the AIDS education scam lobbyists ('loveLife') who play into the hip-hop (here, 'kwaito') culture;
* stigmatisation of HIV+ people is so fierce that ordinary people are scared to get tested, to declare their status, to talk about AIDS, or to do activist work;
* some reports of dry sex traditions which cause much worse vaginal abrasions;
* some reports that disreputable traditional healers and various hucksters still claim, 'sex with a virgin cures AIDS'; and maybe most importantly,
* life here is hell of a cheap, what with unemployment having risen from 15% to 30% since liberation in 1994, and with black incomes down 19% in real terms according to official gov't stats (and white incomes up 15% thanks very much), leading to worsening crime and various social pathologies... and so, incentives to have safe sex, and indeed, to even just live, are not so attractive these days.