There isn't, you're right. For me it was also important when I learned that if you had Y chromosome without (or with, I can't remember) certain genes then you would develop as an AFAB person. And that you could have different genitals without anything uncommon in terms of chromosomes. Or a that our hormones certainly don't work in black and white manner. So I wholeheartedly agree that sex is not binary, and we can easily prove it scientifically.

But I don't argue if someone says "well nature intended us to be male or female" because I don't know that. It might be true. Might not be. I'm willing to give up that argument because we don't really know the purpose of that variety (I googled but couldn't find anything, tho again maybe I'm missing something) but sex still isn't binary. Even if it's intended to be 2 it simply isn't and this happens among all the other species too. I'm not trying to sugarcoat but I genuinely couldn't find anything to counter that and I feel like it does make a lot of sense (at least for me)