I read something, probably here, stating that many black men who are bisexual or homosexual, tend to hide their preferences from the rest of the community. In other words, they pretend to be straight. If they are HIV positive, they can certainly pass this on to an unsuspecting female partner. As far as black women being the largest growing number of AIDS patients, I haven't heard that statistic.
We all know that statistics can be manipulated ;-)
Your liberal friend probably has unimpeachable statistics and is quoting percentage increase. As an example, suppose in a city there are 500 gay men with AIDS and 1 black woman with AIDS this year, and next year there are 600 gay men with AIDS and 4 black women with AIDS. There was a 20% increase among gay men, but a 300% increase among black women.