dfranke didn't make a "correct" assumption, he made an "unnecessary" assumption.

I should have included "if he wished to gender his pronouns". I meant to communicate that the assumption he made was the correct one given his information and priors at the time; I grant that it spilled over into saying that gendering his speech was a correct choice and I did not intend that.

I find it really quite surprising and disheartening that the Less Wrong community doesn't have an interest in making a habit of avoiding these

Actually we do - as I said in the previous comment we are partial to this practice, but it is not (yet) a community norm the way that, say, having read the Sequences, or arguing in good faith allowing for the possibility of changing your mind is. I fully expect it will soon become a norm.

A note on indignation: although it's a greasy social psychology point, indignation isn't the correct response unless it is a community norm. Reacting indignantly to something which is normally reacted to neutrally or ignored marks you as the unreasonable one, instead of the person that casually insulted you. Of course, this is only where "correct response" means "response that achieves the goal you want". (There's another interpretation of "correct response" that would say that indignation is a correct response, and that it fails to achieve the goal you want is a fact about the environment, not about the response).

if the community assumes that everyone in the community is male, then the community is more likely to lose female or third-gender members

Given the concern that LessWrong already suffers from style and interest deficiencies in such respects, this is a crucial matter. I don't know how to address it other than to increase my efforts to avoid gendered speech and more often point it out to others.