...that excluding trans women from all female spaces harms cisgender women too. We're setting up a system that does not actually make us safer (because trans women as a whole are no threat to us) - it's more likely to help men discriminate against us eventually. Always be suss of men who act like we're delicate little flowers who need to be protected from whatever Big Bad they're focused on now.
I mean, I wish people would just accept trans women because they're women. But even if one is struggling with this...going along with the "We Must Be Protected From Evil Transgender Women Who Are Actually Just Men Who Want to Hurt Us"? It's not going to lead anywhere good for cisgender women either. For me, on a gut level, it FEELS like how white men often seem to expect me - a white woman - to be scared of Black men. I hate that, I've never bought into that (possibly because the biggest actual threat to my physical and psychological safety in my life has been my white father, so the scare tactics fall a little flat), and I don't buy into trans women being a threat to me or somehow taking something from me as a cisgender woman either.