I'm not sure sure why it's insulting. I didn't say gender wasn't personally important to people, or that people are deluding themselves or anything like that. Or that it's entirely determined by outside forces and people don't have their own gender identities.
A thought experiment: you were born aboard an alien spacecraft. There are no other humans in your experience. The aliens don't look like you, but know you are intelligent and teach you their language and treat you mostly with respect. They don't have the concepts of masculinity or femininity, and so you've never heard of either. Nobody ever told you about gender, gender roles, gender presentation, and so on. Nobody has ever called you a boy or a girl, or told you you're not supposed to do certain things because of gender, or treated you with more toughness or gentleness, or more dominance or competitiveness, because of gender. So what is your gender identity?
If gender is not a social construct, why do gender roles and rules of presentation change so much as society changes? Why do some societies have two genders, some three, some four, some more than that? Why did transgender, as an identity, not exist before the 20th century, and genderqueer not exist until the last few decades?
I've said before that gender is like consciousness -- it isn't a thing that exists, but it's a thing you experience. And it's a thing that is reinforced (or its wrong fit emphasized) by others, consciously and unconsciously.