Of course not. IV drug use, etc are factors. But living on the downlow is a lot more prevalent than most people think.
When I was living in Los Angeles, I knew quite a few "heterosexual" black guys, mostly athetes on various levels, all of whom had wives or girlfriends but liked to "have " every once in a while.
None of them that it was a big deal, nor considered themselves gay or bisexual. And none of their wives/girlfriends knew about it. I even know a girl who is HIV-positive and has only had sex with two partners in her life. Unfortunately the second one was on the downlow and had HIV.