How is this anything but a non-sequitur? Personal attacks aren't ungrammatical and racial slurs aren't ungrammatical. Yelling "FIRST!" at the top of a comment thread probably isn't ungrammatical either. Like using 'he' for a generic user, each behavior is obnoxious (albeit in different ways and with different scales).
It is true that the use of male pronouns in gender-neutral context has historically been considered correct and is still considered correct by many. It is, however, generally discouraged because it only serves to reinforce the very real and problematic implication of male as the "default" sex.
Incorrect. Grammar, like all aspects of language, are socially generated. Currently, the shift has been to move towards 'she' or 'they' as a default gender.
Saying "'he' is grammatically correct" is like saying "C++ is the proper programming language." It's all about usage.
Unless of course there is something on this service that actually limits all of your users into being one gender :)