The universe holds immutable truths: matter cannot be created or destroyed, an object in motion will stay at motion unless acted on by an outside force, and any time a woman points out sexist bullshit, some hero's opinion will gallantly ride in on a fedora, chiming in "Not All Men!"
Now, thanks to the internet's collective sillybrain, Not All Men! has gone from an irritating trope to a funny, giddy skewering of point-missing folks whose knee jerk reaction as part of a privileged group is to defend themselves against implications that they, as members of the complained-about privileged group, might be complicit in the status quo. It's defensive bullshit that doesn't really do anything but prove the bearer of Not All Men is more concerned with saving face for themselves than, you know, actually acknowledging the concern that another person is expressing. In the Not All Men mind, it's worse to be called sexist than to actually be a victim of sexism. Here are some typical places to find Not All Men in the wild: