The concept of binary and static gender is as much "basic biology" as three states of matter are basic physics or numbers being one-dimensional is basic maths.

It’s an extremely simplified and archaic breakdown because 13 y/o usually aren’t smart enough for the actual uni-level stuff and the way we’ve seen these things for centuries usually are a good introduction to the topic.

Transphobes coming with the "it’s basic biology" just shows that they’ve mentally never progressed further than the 7th grade; that they’re either unwilling or incapable of grasping the fact that the world is more complicated than we tell teenagers.