One model of gender is that you are the gender that you identify as. This is a great model because everyone gets to be the gender that they want to be. It is, by definition, desirable for people to get what they want.1
Many people express concerns about the self-identity model of gender. What if someone abuses it? What if someone disingenuously claims to be a woman, not because they want it, but for some strategic advantage? I don’t think this is much of an issue, since empirically this does not appear to happen very often, if at all. But the concern is noted.
I think a far more common issue is when someone is deciding what gender to identify as. You can’t use the self-identity model as a guide when your self-identity is precisely the thing in question. When the self-identity model works, it defines what is desirable, and that is well and good. But it’s necessary to find at least one other model to supplement it so we can understand what it is exactly that we’re desiring. Here I will briefly describe a few alternative models.