Agree with you given my experience. Kids question many things (religion, cultural norms, fashion norms, etc) and gender is now something that one can question, so it happens. And why not? People have been wrestling with gender roles for centuries, from questions of royal succession to whether women could inherit property or not to suffrage. Yet gender roles continue to be rather prescriptive. What intelligent kid wouldn't dig into why? And why would they take the thoughts of "the olds" as the last word when youth have never done so in human history?

So do what you'd do when your kid is rebelling against their religion or curfew or your selection of their college major. Talk about it and ask what's up. Express your own views, respectfully. Listen.