Wellllll, it's certainly easier if you're not moored to biology.

But if you want to moor it to potential biology, this is what you do.

(1) Our functioning as people is primarily a function of our brains not our bodies.

(2) Many people are socialized to and/or instinctively accept a particular sort of congruence between their brains and their bodies. We recognize the two most common as "men" and "women" (and their bodies have different morphology: "male" and "female").

(3) It is possible to have a discongruence from the normal pattern, however. The specific discongruences of "woman-brained but male-bodied" or "man-brained but female-bodied" are the classic transgender situations.

(4) Because the fundamental experience of who we are and how we act is based on how we function, not what we're made out of, and "trans women" are actually "woman-brained", "trans women" are "women". Likewise for trans men.

(5) Because we can't read minds, the only person with sufficient access to know whether they are "man-brained" or "woman-brained" is the individual with that brain. Therefore, being a woman (trans- or otherwise) is a matter of personal report, not external judgment.

The way I've phrased it, you can dial things all the way from 100% nature to 0% nature and get the same answer: trans women are women, no matter what.

There are some steps that are pretty dubious, but it's not just flat-out patently absurd like "sex is a social convention" is. It can in principle be tied to reality, if reality works out the right way, and if reality doesn't work out quite the right way, you can still maintain something close by weakening the conclusions only as far as reality forces you to.