Thanks for the responses. Apologies for not replying individually. A few things.
Re Butler and 'You just have to redefine what a woman is.' ...
I do not see how that can help. At best it just changes the subject. After this redefinition, we will have two meanings for 'women': let us call them 'women' (already extant meaning) and 'women2' (new, redefined meaning).
[This assumes, contrary to fact as far as I can see (and I have looked!) a redefinition that manages minimum requirements of self-consistence and non-circularity.]
OK, now, if 'women' means 'women2', 'transwomen are women' can be true. But why should I care about that? (Apart from it being potentially seriously misleading.) It leaves untouched the simple fact that 'transwomen are women' is analytically false if 'woman' means 'woman'.
That is at best, as I said. It is not what Butler and her ilk do, though. Without going too much into postmodern equivocations and the like, just notice the dependence on things like 'Butler's acceptance of the body as a historical idea'. Bodies are not historical ideas.
Perhaps there is a redefinition of 'body' ... But, again, why should we care? When I wrote just now, 'Bodies are not historical ideas', I meant 'bodies', not 'bodies2', whatever that might be.
In the end (or, perhaps, thinking of postmodern so-called thought, from the very beginning ), the ideology we criticise here does not make sense.
This might have strategic implications. Is it really worth marshalling scientific knowledge, statistical fact and so on, against an ideology that fails in its very beginnings to make any sense at all? I wonder. Perhaps a response to 'transwomen are women' might be, not 'No they are not ...according to science ... etc. ' but, rather, 'What on earth do you mean?'
Of course it may be necessary to tackle the consequences of the nonsense in practice. My (our) heartfelt thanks are due to all of you who have engaged politically, legally, socially in such practice. Somewhat to my surprise, it turns out this matters, something like the way other such nonsense like, say, transubstantiation used to matter. Let us hope transgender goes the way of transubstantiation and stops affecting the lives of those who see it for the foolishness it is.
[Just in passing, my boast/confession of my age (elderly) and sex (male) was occasioned by the title of that earlier thread begun by the soi-disant 'young feminist'. I hope that does not make this a forbidden thread about a thread.]