“Not all men” has now become the kind of internet phenomenon that generates exegetical click-bait at news sites for the unhip cognoscenti. The underlying truth-conditional quibble is something that has been known in the natural-language semantics literature for decades: the generic force that often attends the use of English bare plural noun phrases is not semantically equivalent to universal quantification. “Lions have manes” (not *all* lions!); “Humans give live birth” (not *all* humans!); etc. Men who object to generic assertions on the basis of their non-universality are seriously missing the point (not *all*—er, yes, all of them).
For more, check out this concise overview of generics by Jeff Pelletier, or go and read Greg Carlson’s dissertation, or get yourself a copy of The Generic Book.