And, whether or not you put a gender designation on a birth certificate means nothing. Males (even transgendered males who have not had extensive hormone treatment) are sexually attracted to females and females (even transgendered females who have not had extensive hormone treatment) are sexually attracted to males. Whether or not actual sexual assault occurs in these mixed gender bathrooms is immaterial. Plenty of sexual harassment will be occurring (I know from personal experience that it has) and people who value modesty will be forced to hold their urine until they get home. That is discriminatory in a far more far-reaching way than the tradition segregation of bathrooms based upon biological gender.

And, by the way, transsexuals who have actually had their gender reassigned by surgery and hormone treatments can get their birth certificates amended to reflect their new status.