if I was a female rationalist it would be offensive to me that Eliezer assumes I would respond differently to the same comment simply because of the gender of the commenter.

And you'd be wrong to be offended.

Because as far as we know, humans can't reliably switch off the biases that would make them act irrationally in such a circumstance, and a rationalist should be humble enough to acknowledge when his/her brain can't be expected to do the right thing.

That being said, I agree with your second paragraph: there's nothing wrong with making generalizations, per se. (Actually speaking about them, however, or otherwise revealing them to other persons, alas, is fraught with many perils.)