I hadn't heard about the party motif - but years ago the terms "Bug Catcher" (one who deliberately attempted to acquire AIDS, and "Gift Giver" (the one that passes the virus on) were introduced to this fetid side of culture.
And at the time, the ONLY thing I could figure out was that they were attempting to elevate the profile/demographic of AIDS victims in an attempt to get more research dollars. However, given the depraved, sociopathic behavior that goes along with that crowd...the mere "thrill" of taking the risk must add a level of excitement to an already dangerous lifestyle choice.
And it could be a "sexier" way of eventually killing yourself. Going out, literally, with a [back-door] bang.