Consider, for a moment, Kohlberg's theory of moral development. I have several arguments with it, but I think it does capture a few important truths, and I think it's a useful tool for conceptualising why people adopt their moral stances.
Now, consider the way (mainly American) conservatism has framed the concept of "Political Correctness" to mean a type of group-think hewing to the party line on progressive issues.
They're right - it exists.
The fatal flaw with their argument, of course, is that this does not invalidate the stances labelled "Politically Correct", nor does it mean that everyone agreeing with those stances does so because of group-think. In terms of Kohlberg's theory, people can follow the same ethic whether they do so because of stage two moral development ("I get something out of it"), stage three development ("it's what all of us good boys and girls believe") or stage six ("It's consistent with that which is right").
Consider a possible analogy - the abolitionist movement in the States. Someone might have been an abolitionist because their spouse was one and they sought domestic harmony, because everyone in their circle thought it was a good idea and they went along with the crowd, or because they had intellectually considerd a set of ethics from which human equality genuinely derived. But pointing out that abolition was "politically correct" didn't justify slavery.
The test for these positions would be dissent, especially dissent within the ranks. Consider someone who stated that slaves were trained to be subservient and therefore very few of them could every really be freed. If you were an abolitionist for post-conventional reasons - because it was right based on your system of ethics regardless of what other people said - you might consider this proposition on its own merits and engage with it. If you were an abolitionist for conventional reasons - because it's what all good people believed, or because it's what your peer group believed in, and you were obliged to go along - then you might consider this proposition a threat and attack the person who made it or their motives for doing so.
Indeed, in the latter case, we would expect to see a certain dynamic showing up. The person making such a proposition would be vilified, and their right to speak would be questioned. If possible, it would be taken away from them (*). Their actual argument would be distorted as people reacted to what they thought he or she was standing for, rather than what he or she was actually saying. And people would engage in Two Minute Hate sessions against them to reinforce group bonds around what was Correct.
These, I submit, would be clear signs that a position was held because it was conventional for the group the person identified with rather than due to actual moral consideration, regardless of the validity of the position.
These are obvious behaviours on the wingnut sites. But the wingnuts are right - you don't have to go too far to find the same on progressive sites either.
(*) I've gone through three copies of James Carse's "Finite and Infinite Games", and now I can't find that third copy! I suspect a certain goth friend is sitting on my third copy - if he isn't, I'm going to have to go shell out another twenty or thirty bucks for yet another copy. Read up on what Carse has to say about "Evil" if you can. Hell, read the whole goddammned thing - it's an excellent book.