There's an instructive and enjoyable post here by Pascal Boyer, in which he exposes the mechanism of a widespread but useless form of intellectual argument. I know it as: there's the bit where you say it and the bit where you take it back.
A familiar form of this is: 'There's no such thing as human nature.' 'Really? But what about the various biological constants, and the human needs and abilities that are based on them?' 'Well, there's obviously a human nature in that sense; but I meant that human beings aren't all greedy, selfish and devotees of reality TV'. Net result: denying at first there was any human nature, your interlocutor ends by arguing that some particular qualities (usually ones she disapproves of) are not a part of human nature.
Pascal Boyer presents some other examples of the method and names it differently:
[C]onsider relativism, which tells us that people literally live in incommensurable worlds. Or the common anthropological idea that kinship has nothing to do with reproduction and genetics. Or the literary critics who say that writing is primary and orality is a derived form of communication. Or the notion that gender is completely unrelated to sex.
The mechanism that made these strange notions popular is actually not so mysterious. It is a simple variant on the age-old technique of bait and switch, that is, giving the sucker something attractive to expect (the bait) and then substituting a dud once the customer is hooked (that's the switch).
Most of the academic ideologies I mentioned, and I imagine many others, are attractive because they seem to violate some of our common assumptions... Manhood has nothing to do with being a fellow! One is not born a woman, one becomes a woman!
But on closer inspection, it generally turns out that the initial, amazing, challenging statements in fact disguised crashingly banal assumptions. Suppose you point out to your academic ideologue that, for instance, if maleness and manhood really are completely unrelated... then it is puzzling that an extraordinarily vast number of [socially constructed] "men" happen to be [chromosomal] "males", and that such a coincidence is spooky. You will probably be told that you did not quite understand the original statement. What it meant was that the meaning of maleness could not be derived from possession of the Y chromosome...
It's the banality of the preposterous.