Should I simply ignore it if authors assume that I'm male in their response to my review of their article?
As a reviewer, you are supposed to comment on academic value and scientific correctness of the manuscript.

As a woman, you feel unhappy about authors not guessing a correct pronoun for you and not using an appropriate gender-neutral pronoun. 

It seems that the issue has nothing to do with the manuscript and hence you are not reacting with your reviewer hat on. You are considering your response based on your role as woman/feminist/activist &mdash; but not as a reviewer. You are not reviewing the paper, you are reviewing author's communicative behaviour.

As long as you make it clear that you are not commenting as as reviewer of a manuscript, I think you can make this remark to the editor.