If I may?
My guess is that Horrorble is using the systemic context of the word "sexist", while Aquitaine is using the individual context of the word "sexist." There is no doubt that people across the board can hold prejudiced views and biases according to what they have learned about other people. However, I think the roadblock can be understood as the distinctions between the scope of the impact of sexism.
So, perhaps Horrorble isn't suggesting that men in a vacuum will exhibit sexist thoughts and behaviors, but that she is suggesting that men in an environment that is systemically pervasive sexism against women will - like everyone else, including women - fall in line with the status quo.
I think Aquitaine and others are taking Horrorbles assessment of a patriarchal social structure as an attack against men. Having enough conversations with her at RF, I don't believe that is her intent. I think her assessment of men living in a patriarchal society is that she believes men are unaware of how they perpetuate the status quo.
Given enough time, I'm sure she can point out how women perpetuate the patriarchal status quo, too. The patriarchal status quo, by its very nature, is sexist against women.
I apologize, Horrorble, if I have misrepresented your views. I felt you were being misunderstood, however, and that is why I jumped in.