Policing the word queer is pretty common TERF rhetoric, so like, unsurprising.
Unless you were hinting at that, then this is me being bad at subtly...
You are asking for their SEX not their GENDER. They are non-binary which means that even IF they were assigned female at birth (afab) they are not a woman.
And yes it is rather offensive because demanding to know their sex makes it seem like you think that genitals are more important than how they identify. Which is gross. I hope you don't treat real people like this.
This is what what sensitivity readers are for, they're just someone who can catch things like this because they have a different viewpoint and background. I doubt some of the people who are asking for one want to make your work bland or easily digestible or censored. It's honestly just a good way to get another perspective, which, I think, is very important in any kind of storytelling.