Actually, science has been solidly on the side of “gender is a spectrum” for a long time now. Gender is also defined medically as related to, but separate from sex. Sex is a biological state of being male or female, whereas gender is an individual’s experiences on a spectrum which includes man and woman. Sex is about our bodies (or more specifically, the alignment between chromosomes, genitals, and hormones; defining it by chromosomes alone would be absurd because that’s not the judgment doctors use to assign sex at birth and not something anyone knows 100% without genetic testing, which is too expensive to give every infant). Gender, on the other hand, is more a mashup of biology, identity, and expression. Of course, gender may also have a genetic component; identical twins raised separately, for example, are more likely to both be transgender than two adopted children raised together. Twin studies aren’t perfect due to how rare identical twins are to begin with (creating a limited sample size), but it’s a good way to reveal a potential relationship.
That being said, sex is also a spectrum. Intersex people exist and are individually viable, though not typically reproductively so. Some would argue that they just have medical conditions, but these people are best respected as intersex because coerced medical interventions in infancy are more harmful than their conditions themselves (almost like we shouldn’t conduct surgery on peoples’ genitals without their explicit consent just so we can fit them into boxes we’re more comfortable with!). Also because, you know… they’ve asked us to respect them as such.
Anyone who’s progressed in their scientific education beyond high school biology class (which is intentionally oversimplified and outdated because they want everyone to pass) at least has some clue of the amount of nuance involved here. Claiming that gender is binary and biological dismisses the amount of complexity involved in humans, especially when even biology (and literally all organic matter in existence) has never functioned as a binary system. As for claims that nonbinary adults “confuse some children”, that has yet to be seen, though it hasn’t been formally studied to my knowledge. From what I’ve observed, children are actually more able and willing to accept transgender and nonbinary identities than a lot of adults. They’re not stupid; they can understand a lot more than you give them credit for.
Overall, regardless of what you believe, it costs you absolutely nothing to respect people for what they want to be called. So if you can’t acknowledge actual, up-to-date, nuanced science (or other cultures, since there are plenty all over the world that recognize more than two genders and have done so for centuries), please at least act like a decent person if you ever meet someone who is nonbinary.