OK, so you recognize intersex people. Good. Let's start there. So we can have people who appear like men or women who actually have the genitals of the opposite (or both), right? OK, so what caused that development? Usually it's related to chromosomes, but that isn't actually the cause. The thing that creates the differentiation is what hormones they have. The chromosomes usually are what controls their output though, so it's correlated.
OK, so we recognize that hormones are the thing that actually causes this. What happens when we artificially control what hormones are in the body? Does it matter what could have happened if we subvert that and control it manually? Which part is biologically deciding their gender? Isn't it the thing actually being expressed? If that's the case, then aren't they biologically women?
There's more to biology than you learned in your high school bio class (that you probably failed). "Basic biology" is, as the name implies, basic and not a full understanding. Anyone appealing to "basic biology" is admitting they don't actually understand any more than that.
(Just FYI so you can know where I'm coming from, I'm a cisgendered straight white man. This doesn't effect me directly, so I'm not arguing from self preservation. This shouldn't matter, but some people would probably discount the opinions of trans people as "arguing from emotion" or some bullshit just to ignore them.)