Of course there are sexist bands within heavy rock. But then again what music genre doesn't have that? Rap, the commercial big stuff extremely sexist just like most commercially succesfull pop.
Heavy Rock not being the commercial powerhouse of the Eighties any more hasn't got to do with being sexist, or actually it does. Since loads of bands started shedding their sexist lyrics Heavy Rock has become less commercially viable. Which mainstream music listener wants to hear about the deplorable state of the world or contemplating suicide or toal nihilism? Not many, I dare say.
Btw whey do people keep on saying Black Sabbath were satanic, when in fact they have never been satanic in their lyrical approach. Yes, there was some horror in corporated in some of their lyrics, but that was of the Hammer House of Horror variety. Sabbath were on about drugs and the sate of the world (just take War Pigs) at that moment.