Are you saying that <I>all</I> the women who have acquired HIV from their female partners happen to be closeted, and are therefore making up stories about using drugs or having straight sex? Anonymous sex, presumably, because otherwise the CDC would try to track the fictiotious male partners down, to find out if they're infected...
It's possible to be very, very bright and still not quite have a grasp on the real world, whether through a lack of awareness of the issues or just sheer paranoia. Unless you're leaving out the part where your cousin's partner <I>has</I> HIV/AIDS, and she's concerned about getting it... but to worry about any given partner, particularly if they're doing the sensible thing, and getting tested for STDs before they start having sex... I mean, first, her partner would have to cheat on her, probably with a man. She has to cheat on her with someone who's infected, at least, and either not take proper precautions, or have them fail. She has to cheat on her with someone who's infected, <I>get</I> infected, and <I>then</I> transmit the virus to your cousin... which has, as far as the reports I've cited go, an exceedingly low probability in-and-of-itself. Add it all up and I think it's approaching zero... does your cousin have trouble driving, because she's afraid of being in a lethal car accident? Because it's probably more likely.