Because of how they would have to be policed.
This is where you seem to be missing the point jj. What you're doing when you say stuff about toilets having to be 'policed' is assuming that when there is a rule about males not being allowed to use women's toilets, some males will try to use them anyway. Probably some would, but what sort of person would do that?
You're making the assumption that 'transwomen' will try to use women's toilets even though they know that they shouldn't, so you're showing that you don't believe 'transwomen' are decent people who will abide by the normal rules of society, but that they are unpleasant people who will try to force themselves into places where they know they shouldn't be and where they are not welcome.
I'd say that's somewhat transphobic of you jj.