Some still do. Poppers can cause immune system collapse in the exact moment you take them, the theory being you're much more likely to be infected at this point. Other theories are/were previous exposure to an STD like syphilis (i.e. it's either mutated syphilis or just the disease damaging your immune system). Or just hard drug use, i.e. cocaine and heroin that leaves your immune system more susceptible to the virus. And then there are still even crazier theories that abounded and are still around like HIV and AIDS are completely unrelated and the drug that people take for HIV actually cause AIDS etc.Indeed, in fact they thought poppers (amyl nitrate) was the cause - and that has absolutely nothing to do with fluid exchange or Crisco.
Perhaps the worst part of the early discovery of the disease was that it only affected Haitians and then gay men, and thus for a very long time it was a curiosity rather than a disease to be battled by governments (Reagan famously dismissed the AIDS threat and a lot of people blame him for allowing so many people to eventually die).