You are hardly being petty. I'm genderqueer myself and I would have difficulty explaining that to people who are overly focused on what one's physical sex is.

I'm sorry you had to go through that. My suggestion for future times for when you have to do character creation and they force you to chose a gender, invent a story concept behind the avatar and work with that. You don't even have to tell the others. Hell, you can base it off a character that you're already familiar with. That's what I often do when I do character creation.