One problem with this simplified model is assuming every sexual act is with a new partner, which would only be true in the very early stages.

It assumes that every sexual act is with an uninfected partner. Perhaps that's what you meant, but then I wouldn't have used "very."

If you consider that all other STD's would infect this population before HIV, then one has to wonder how that would effect condom use, and how that changes the model.

I think this is pretty well documented. STDs were routine and not a big deal (treatable!) in the 70s gay scene. Thus they did not cause condom use.