Being a feminist is incredibly hard.

In the same way being a rationalist is hard (i.e., it's cognitively difficult.)

If a man at a LW meetup said something objectifying, misogynist, or racist, would another male LWer call them out for it?

My mental model says yes, and I have actually seen it happen twice.

I doubt most of them would speak up

I think this is strictly correct, but I predict better outcomes in the case where one person says to the offender "Hey, that's out of line" than when everyone in the room castigates the offender in unison.

imagine the irony of a "feminist" man telling a woman what feminism is!

Maybe you meant "what oppression is"? (And maybe that would be a helpful distinction to make in general.)

arguments about feminism or anti-racism on Less Wrong commonly come to a feminist stating some widely-accepted notion from feminist theory or feminist psychology, and a LWer stating that the notion must be false because of some intuition.

Do you have some examples for this?

I generally agree that there are problems with how SJ issues are addressed around here, but my experience as an Actual Female On LW doesn't correspond to the picture you are painting of it being a bunch of intellectually snobby dudebros.