That kind of statement is particularly annoying, above and beyond considerations of its truth value, because it tends to come across as judgmental: "Real" men/women/rationalists/whatever do X or Y or Z, so if I don't, does that mean something's wrong with me?

Yah; it comes across all too often like a retroactive attempt to patch an idea that might be compromised by bias. Especially because those minority of cases may be the real salient test of the idea -- if your theory is predicated on the idea that all X are Y, and along comes an X purporting to be a Z but not a Y, then conditional on the truth of this statement your theory is wrong. It's one thing to look at the failures of your original formulation and go, hmmm, clearly I missed something and need to patch or reject my theory; but in a context like this it's usually more, well, a rationalization -- "your counterexample doesn't apply because my factual error can be retconned as a previous, weak definition of the scope of my statement!"