The biggest problem transwomen have is their own maleness. It causes them great misery and in some cases to have complicated, difficult surgery and take lots of medications.
Women also have a problem with the maleness of transwomen, because transwomen are being encouraged to go into spaces where women are vulnerable, as if they were just another woman, but transwomen are male (see above), and maleness, full stop (not just in transwomen) is correlated statistically with increased risk to women.
What we have now is a situation where, because transwomen themselves can't bear to think about or refer to their own maleness, other people, including women, aren't allowed to either. Women therefore aren't allowed to describe this particular subset of males as males, even though they themselves and everyone around them know perfectly well that they are males.
In lots of ordinary minor social situations there's no need to refer to the maleness of transwomen even if it's blindingly obvious, and it's courteous not to. But when making decisions for women as a whole, e.g. about who should be allowed into prisons with us, who can compete with us in sport and so on, we have to be able to use accurate, truthful language and describe this particular set of male people as male.
SergeantDawkins - I get it, years ago I used to think that calling a transwoman male was the worst and most horrible insult, and surely no one should do that?! But the problem is that not being able to accurately describe a male person as male puts women in a very difficult position. The reasoning behind single-sex spaces staying single sex is to protect women, specifically from dangerous males. The reasoning behind transwomen staying out of those spaces is not because they're trans but because they're male. No amount of saying "but transwomen are women" erases that maleness, much as they wish it would.