"How do you not understand yet?
If you let in anyone that says they are a woman, you let in ANY man."
Let in where?
Are you talking about toilets/changing rooms?
You think if we don't pass laws to ban trans women from women's spaces (and presumably have these laws policed by gender checkers on the doors) we'll have predatory cis gender men capitalising on this by constantly pushing into women's spaces to assault women under the guise of 'I'm allowed to do this because I identify as a woman'?
Because predatory cisgender men pretending to be trans to predate on women has been a significant problem up to this point? 
This reminds me of 'we need to stop immigration from Muslim countries because otherwise the UK will soon be overflowing with Muslims forcing sharia on us, and gang raping children all over the place.