well, it seems that here we have the one and only fully-documented case of HIV transmission between women, and it turns out to also involve the exchange of blood which, of course, is the easiest way for anyone to transmit or receive HIV, and is a fairly easy risk factor for an HIV-positive woman (or her partner) to avoid.
the 'rule' is that it is incredibly unlikely for women to transmit HIV during 'normal' sex, which seems to hold.
i guess the judeo-christian god is exactly as good at designing plagues as he is at designing humans. or maybe He's just into lesbians? i don't know.