Unisex changing rooms with lockable cubicles such as we have locally is the way to go. You should always have the option of not seeing someone's junk, and for other people not to see yours. That deals with the issue of people who don't want to see a particular set of a genitals.
It 'seems' like an answer, doesn't it? But all that does is severely curtail women's current freedoms.
For example, my gym changing room does have some cubicles (which may women prefer to use, rather than strip in the inner, communal area. They still have to walk, naked apart from a towel, to the showers, however. In the 'communal cubicle changing rooms', many women (and I'm one) would be extremely uncomfortable changing in proximity to an area in which men might be naked and many (and I'm one) would be unable any more to use the showers.
I think what many women of the 'I don't see a problem' variety are missing is that (a) there is now a significant number of men declaring themselves trans who have not the least intention of physically transitioning, (b) some of these men are already physically violent towards women, (c) self-id offers predatory men (who may well not be trans at all) to enter women's safe spaces and - as others have already said - know that they cannot be challenged.
Those saying 'some transwomen are already using women's changing rooms and there's been no problem' are closing their minds to the potential threats I've outlined above - it's not those transwomen we're particularly worried about.
The 'what about lesbians' comments are apparently based on the notion that lesbians are as much of a threat to other women as are men. They're not.