I don't buy the argument that "fill in the blank-ism is the combination of prejudice with institutional power" argument (which I'm sure makes me a pariah around these parts). It may be weak and toothless "ism" without said power, but it's still not right and is unjust. A blanket judging a person by race, color, sex, gender, religion etc... is still a pretty a-hole thing to do even without the power to force anything based on that judgement. It may not fit an academic definition of a particular "ism", but I believe that it fits a commonly socially agreed upon definition.
A comparable situation would be a lot of racism perpetrated by "minority" races. Just because a target may be white doesn't mean that it's not racism and is somehow excusable or justified (and before anyone crawls up my ass on this one, I am a non-white "minority").
And you'll have to excuse my "penis" comment. No offense was meant by that, and while I acknowledge the "cissexist existentalism" of the statement, it was meant merely as a somewhat dismissive note on how ridiculous it is to judge the worth of people by their genitals (or perceived gender or whatever). I'm not going to go back and edit my statement, but perhaps I should have written "as someone who is perceived by society to be a heterosexual male".
Seriously, to boil it down to it's essence the thought is "Hey, singling out a group of people for ridicule or persecution is kind of uncool regardless of what said group's power status is in the current society". Simply put, it's not constructive and rarely (if ever) leads to a good place.