But unions will help you in some cases, and for women not in others. So if you think you will get value from your subs why would you leave? there is a LOT more to being in a job than fighting the genderwang.
And then, of course, if you aren't in the union you can't (try to) force them to change. So there are very good reasons for holding your nose and staying in.
It's the issue i have too, and i live in a country with SelfID laws so i would be fucked anyway. But my union have just helped me with a redundancy, i would have been in big trouble without their support. So, for now, it is worth me staying in.