You're making incorrect assumptions about this "decision". The thread title and OP's comments are not the result of the child making a conscious decision, they're comments from an observing parent. What the child is doing is simply what feels natural and without are understanding, or lack of in some cases, or biases, or millions of references we have in mind, which they clearly do not.
Now you're talking about biology, as in, sex. I'd recommend taking a minute to go read about the difference between sex and gender.