Because women are perceived to be the weaker sex therefore it is rude to argue against them. Most people don't want to be seen as rude, except actually rude people who don't care. It doesn't matter if MRM have a point, they will inevitably be both seen as rude and actually have a disproportionate number of rude people.
I think you're correct about it being rude. More than rude, it's a social taboo to criticize feminism. The statement "women are perceived to be the weaker sex" does not seem to generally apply. It's more that we've internalized the more that "Anything that looks like an attack on the concept of equal rights is to be shunned." That gets extrapolated to "Anything that looks like an attack on the tools we've used to get more equal rights should be shunned." Note that the latter is not a position I endorse.
It's complicated. ...
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