The argument is that the best way to protect trans people is for everyone to pretend they're literally the opposite sex. But people tend to not go along with this when it's stated openly, so they get there via the sleight of hand that is 'gender identity'. Unfortunately, trans rights activists are, with the collusion of the medical industry, simultaneously vastly expanding the category of trans to accord with this unfalsifiable object, 'gender identity'. In practice it is a subjective mental state evidenced by an act of declaration. Again, people are unlikely to go along with this (especially the transing of children) if it's stated directly, so there is more obfuscation. Obfuscation works better when combined with threats, accusations, and silencing, though, so the movement has adopted much of the language of social justice movements to make their goals seem legit and to make critics seem like villains. But yeah. The basic position is that any man who says they're a woman is literally a woman and should have access to all women's rights and spaces, and be legally and socially treated as though they are literally a woman. This is in direct opposition to women's separatism, a crucial tool for improving women's lot in male dominated societies.