Basically it boils down to the simple conclusion that people are nowhere as promiscous as latter day liberal wisdom would have us believe. Just because they are rutting with anything - male, female, animal or mineral - that moves does not mean that we all are.
Even if that assumption were true, for straight people it's not all that much of an issue. Without getting into gory details, male homosexual sex is much more conducive to the transmission of HIV than what heterosexuals do. And don't forget about how dangerous sharing needles is. Even a very promiscuous heteresexual is at a lot less risk than his gay or needle-sharing counterpart.