See, this is what puzzles me.
We all agree that trans women are women, yes? 
If that's the case then the statement 'only women menstruate' is true. How can this statement be offensive to anyone because trans women are women aren't they, and therefore saying only women menstruate includes all women, trans or otherwise, surely?
So how can it be offensive? Who is it offending? The only people excluded from this statement are men, who don't menstruate. I'm lost as to what women are offended by this.