I'm not sure about this but my guess is that a man who refuses to abide by an establishment's rule of "no men in the women's showers" would be asked to leave. If he does not leave, he could be arrested for trespassing.

But given direction these new laws are taking, I'm further guessing that the Police should refuse to remove the man and instead arrest the proprietor for discriminating against this man based on his protected status of identifying as a woman?