That's exactly what the bisexual women do. They're highly promiscuous -- I think there was one study showing that bisexual women who go to the gym have something like five times as many partners as straight women.

So they're sleeping with men and women at alarming rates (probably not as much as gay men sleeping around), and some of those men are gay/bisexual; they catch something from them, and then pass it on to straight men or bi-curious "straight" women.