Good, we agree. So when women see obvious males entering single-sex spaces and using single-sex services, how do we know who the liars are and who are the ones who genuinely believe they are women? (To be fair, my position here is that it doesn't matter, neither group should be there, but some people try to say it does make a difference and the second group is safe - but it all depends on there being no liars, doesn't it?)
Adam, your starting point on this issue seems to be that trans-identification is a given and people can decide for themselves what their gender identity is, which will trump what sex they are. My position is that these beliefs about gender identity are like religious belief (at best). There's no way for anybody else to verify what someone's gender identity is. Many people who are trans-identified seem to be in thrall to gender stereotypes, which is problematic for the rest of us who would like to see an end to that sort of pigeonholing.
Biological sex on the other hand is real and immutable, and it's easy for most of us to identify what sex another human is, as you would expect for all mammals.
So - if we can't even agree on the definition of terms, how are we supposed to have confidence that a confused girl understands what she is saying when she says 'I identify as a boy'? How is it ever going to be safe to put her on a powerful drug that will stop an essential bodily process like puberty and leave her with lifelong side effects, which may include a drop in IQ and permanently weakened bones? How can we expect her to use her brain, which is artificially kept in a child-like state by the drugs, to make the most important decision she will ever make in her life, namely, to go on cross-hormones, which will mean she will be infertile and unable to enjoy a normal sex life, not to mention yet more physical health issues such as increased risk of heart disease and cancer?