If the law allows only people born as women to use women’s bathrooms, then someone who was born a woman but now looks, dresses and behaves as a man can enter women’s bathrooms.
The law opens a can of piranhas, doesn’t it?
It means that anyone who looks like a man could enter a women’s bathroom, and if the women in the bathroom object, then the “man” could say, “Knock it off, I was born a woman.” (I used to write sketch comedy; I can imagine a pretty ridiculous five minutes from this premise.) Do the women have to let him stay, or does the law force him to produce proof that he was born female?
Who’s more objectionable in a women’s bathroom: someone who looks like a man, or someone who looks like a woman?
And if people worry about transgender people assaulting the cisgendered — isn’t it more common that the cis assault the trans?