The real problem is that it's highly intellectually dishonest. Being a person with a logic fetish, it's not actually bad moral premises that piss me off (as much); it's a bad argument. I have a lot more respect for libertarians than for the Religious Right. Why? Because libertarians are honest about what they believe and what the results of that are. The Religious Right invoke a man who said "blessed are the poor", "love your enemies" and "he who is without sin, let him cast the first stone" to cut welfare, engage in reckless warmongering and persecute gay people.
So, what's going on here? Well, part of the problem is that identities are actually beliefs (right or wrong ones), not labels. Most of the time, they are labels, of the form "I am an X"="I believe that [traits of X apply to me]". Nobody will really say that a man who believes in the god of the Koran and not those of the Vedas but, for whatever batshit reason, calls himself a Hindu is a Hindu; he's a Muslim. This may sound like glossing over important differences, but affirming a full and complete separation of belief and identity leads you to some confusing and nonsensical conclusions. And this works the other way, as we shall see.
Why is this important? Well, do my highly tolerant and accepting friends condemn Todd Akin? You bet they do. Now, if what Todd Akin were saying were true and women couldn't get pregnant from rape, he would just be stating a fact and they would be engaging in denialism. But he's not. That what he is saying is false and that he's being a misogynistic asshole is actually completely irrelevant here. Todd Akin says "women can't get pregnant from rape"; this is equivalent to "Hi, I'm Todd Akin and I identify as someone who thinks women can't get pregnant from rape." By saying that Todd Akin is being a misogynistic jerkass- or, sorry, making a moral judgment with regards to his being a misogynistic jerkass, since this is a distinction we do need to make- you are condemning an identity. But you told ME that identities are unjudgeable- and, furthermore, you are judging me for saying that at least some identities are judgeable, a belief I am holding, equivalent to an unspoken identity "I identify as someone who judges certain identities". This is a contradiction.
It's stuff like this that makes me not like postmodernism. I have no problem with analyzing issues from a minority viewpoint- indeed it's often the only way to analyze an issue. But let's not pretend that we can condemn the status quo while letting minorities get a free ride. Or we can- but then we chuck any pretenses at universality and consistency out the window. That's fine with some postmodernists, I'm sure, given that- as per the Sokal Hoax- postmodernism and sociology seem to be content to tell science that they are better at science than science is.