But there are adult women who refer to themselves as girls.
There are, of course.
But, to me, this is where the whole thing gets very muddy indeed.
Because people, including me, do not like anecdote presented as evidence.
But there is a point which anecdotes become so overwhelming that they cannot be ignored.
It is a very specific characteristic for (some) transwomen to refer to themselves as girls. Over and over.
I suspect it is those who are autogynephiles. Because it often goes hand-in-hand with cutesy names like Tiffany.
So 'girls' as collective noun, unfortunately has a connotation that, although you can dismiss, is certainly something of a flag to me.