It's obvious that you have a visceral distaste for calling black people white "in reality", but given that - and this is the point of the damn essay - why do you consider calling women men acceptable?

Because it's obvious from the context that by 'men' I mean human beings. If you put aside the chance that it will trigger certain biases in the reader, there is no reason to feel offended by the use of words like mailman... no reason except the unthinking reflex of political correctness that drives certain people to get hysterical when they read the word 'nigger', or to get offended when they hear Neil Armstrong's legendary phrase (even the version with an 'a').

And yes, if we lived in a world where 'mailwhite' had the same (widely agreed-upon) meaning that mailman has in our world, it would be silly to be offended by it.

I thought that the extension from "implicitly excluding women" to "making women feel unwelcome" was evident.

If there are women who are made to feel unwelcome by my use of 'him' instead of 'them', and similar conventions, they're hypersensitive, because it doesn't implicitly exclude them: I don't mean to exclude them, and anyone who reads my posts will understand what I mean; anyone who wants to understand and isn't looking for an excuse to be offended, that is.

drawing unjustified generalizations about a class of persons

... is wrong because if we value truth (and we do), holding a false belief when it's in our power to do otherwise is wrong. This being the website that it is, we don't need additional justification to avoid such generalizations; there's no need to bring offensiveness into it.