Maybe we should think of it like this: Trans women/men are to women/men as adoptive parents are to parents. There are disanalogies of course, and the morality of adoption is a large issue in itself which I can’t do full justice to here. Still, the analogies are, I think, important and instructive. 
An adoptive parent is someone who desperately wants to be a parent but can’t be one in the normal biological sense. (At any rate usually–there are families with a mix of biological and adopted children. But here I’ll focus on the commoner and simpler case.) So society has found a way for her to live the role of a parent, and to be recognised socially and legally as a parent, which kind of gets around the biological obstacle.
“Kind of”: plenty of adoptive parents report an abiding regret that they aren’t biological parents, and there can be problems on either side of the adoptive relationship. It is clear that the existence of adoptive relationships creates psychological difficulties, both for the parents and for the children, that would not otherwise exist. But these problems are not big enough to make adoption a net bad thing.
One reason why not is that adoptive parents are, in the nature of the case, deeply committed to parenting. Unlike some biological parents, they aren’t parents by accident. And by and large–though unfortunately, adoptive parents do suffer some sorts of discrimination–society recognises and values their commitment, and accepts them for many purposes as parents like any others, though of course there are contexts (blood transfusion, organ donation, testing for inherited illness) where the fact that they’re adoptive parents makes a difference.
Nobody sensible thinks that it’s all right, when you find out that someone is an adoptive parent, to get in her face and shout “Biology! Science!
You’re running away from the facts!
You’re not a real parent!”
That would be incredibly rude and insensitive. It would upset her family. It would be importantly false: there is a perfectly good sense in which an adoptive parent most certainly is a real parent. Yet since this aggressive accusation is also, alas, only too intelligible to the parent who is subjected to it, it would also be stamping up and down in the crassest and cruellest way on what anyone can see at once is very very likely to be a sore point for her. (Here I speak, I’m sorry to say, from personal experience of analogous shoutings.)
Nobody sensible thinks that it’s an infraction of Jordan Peterson’s human rights to impose on him a social, ethical, and sometimes even legal requirement that he call adoptive parents “parents.”
Nobody sensible thinks that, if you refer to an adoptive parent as a non-parent, then you don’t owe it to that parent, as a matter of basic courtesy, to retract, correct, and apologise.
Nobody sensible thinks that the existence of adoptive parents undermines our understanding of what it is to be a parent. On the contrary, it extends it.
Nobody sensible thinks that adoptive parents are, typically and as such, a threat to other parents. Or that they only went in for adoptive parenting as a way to get their hands on vulnerable children or vulnerable parents. Of course, it’s not impossible that someone who is an adoptive parent might be bad or dangerous in either or both of these ways, and of course, it would then be right to protect ourselves and other potential victims from that person. But if that happened, it wouldn’t throw any shade on adoptive parenthood itself, as such.
Nobody sensible thinks that there’s automatically a problem about having adoptive parents in parents-only spaces. There might be some special spaces that should indeed be reserved for biological parents only–pre- and post-natal groups, for instance, or a group like Sands, that helped us when we had a still-born child in 1995. We should be prepared to listen carefully and sympathetically to the case that might be made sometimes for biological-parents-only spaces. But in general, adoptive parents have similar enough concerns and interests to biological parents for it to be, in most cases, both natural and useful to include them in such spaces.
Nobody sensible thinks that adoptive parents are necessarily buying into an oppressive ideological agenda of parenthood, and, by their choice to be parents, imposing that agenda on other parents. There are oppressive ideological agendas about parenthood; of course, there are. But to be an adoptive parent is not necessarily to buy into them. It might even be a way of subverting them.
Nobody sensible thinks that there’s just one right way to be a good adoptive parent, any more than there is a unique right way to be a good parent in general. Though there are some things that have to be in common between all good parents, there are lots of different ways of being a good parent. The broad schema of what parenthood is, adoptive or not, is set by biology and sociology. But sociology can certainly be challenged and often should be (fighting back is called politics), and even biology is not always just to be accepted (fighting back is called medicine). Within the general role of “a good parent”, there is all sorts of room and scope for creativity, self-expression, and imaginative invention and re-invention.
We don’t always know, on meeting some parent, whether she is an adoptive parent or a biological parent. Often there are visible clues and giveaways, or at least we can see things that make us strongly suspect an adoptive relationship. But in most contexts, it would be rude and intrusive to ask. The implicit social convention is loud and clear: you don’t ask, you wait to be told. But when we know all the facts about any parent, we know which they are without any difficulty.
In our society, the role of an adoptive parent is almost completely uncontested. (Almost, though there can be some resistance, and it can be unreasonably hard to get into the role in the first place.) If you’re an adoptive parent, you’re a parent–for most purposes–and no one sensible scratches their head over that, or decrees that you can’t sit on school parents’ councils, or sees it as somehow dangerous or threatening or undermining of “real parents” or dishonest or deceptive or delusional or a symptom of mental illness or a piece of embarrassing and pathetic public make-believe.
On the contrary, people just accept you as a parent, and value your commitment to parenthood as an important contribution to the well-being of our society that you could not have made if you didn’t have the psychological set-up that you do.
We can imagine adoptive parents, when talking among themselves, needing and finding a phrase for parents who aren’t adoptive parents. You can see why the phrase they’d choose might not be ‘real parents’; that would be doing themselves down. (And you can see how that phrase might be weaponised against adoptive parents, by unkind other parents.) Anyway, these adoptive parents might choose, for this purpose, a phrase like “biological parents.” Then language use being what it is, maybe they shorten it to ‘bio-parents.”
Now in this scenario that I’m imagining—for all I know, there are subcultures where it is real—two extreme stances strike me as overdone and unreasonable. One is for the adoptive parents to insist that all biological parents always call themselves bio-parents. The other is for bio-parents to take offence at ever being called bio-parents. Yet apparently, both these stances are actually taken in discussions of “cis”…
“Trans women/men are to women/men as adoptive parents are to parents”, I said. So is that a “subsetting relationship,” to use the philosophical jargon?
Do I mean that trans women are a subset of women, that being a trans woman/man is one way of being a woman/man?
Or do I mean a relationship between two different categories–trans women/men aren’t literally women/men, but they are something closely related, maybe analogically related as Aquinas would say (Summa Theologiae, Prima Pars, Q13)?
Well, it depends on what you want to talk about. For some purposes, sure trans women/men are “really women/men,” just as adoptive parents are “really parents.” For other purposes, the relation is indeed analogical rather than literal inclusion.
But maybe we could follow the philosopher Derek Parfit (Reasons & Persons circa p.262) and say that “once we know all the facts,” the further question “Are they really women/men?” is an “empty question.”
Or maybe we can say what I would want to say, which is related to Parfit’s move, but different: that the question is not empty at all, but it has different substantive answers for different substantive purposes. And provided we keep the score carefully in our language game (s), there’s no reason at all why anyone should be confused about any of the semantic-logical ins and outs of “trans woman/man” any more than they are with “adoptive parent.”
 By the way, I hope I don’t seem to be erasing anyone in this post. I’m writing here just from my own viewpoint as a trans woman. It looks to me like everything I say here from my own experience transfers straight over to the case of trans men, which is why the analogy is about “trans women/men”. Though I’ll be interested to hear their perspectives, non-binary and gender-fluid people probably won’t be covered by this analogy. Though of course they are covered by the general moral rule underlying everything I say here: that people’s life-choices about how they want to be gendered are (like the life-choice to adopt) a deep and serious matter for them and so must be respected.
This article was originally published on 20th July 2018 on the Blog of the American Philosophical Association. Sophie Grace is a philosopher in the UK at the Open University and our sincere thanks for her permission to re-post her blog.