It's not difficult.
You cannot transition from male to female or vice versa. So "female" is not the side they are transitioning to. It is the side we are on and they are not.
You cannot allow harmless trans identifying men into women's spaces whilst keeping the dangerous ones out. Either you keep them all out, or you let them all in, including the Karen Whites and Isla Brysons of this world. Because nobody is standing guard on toilet doors, checking whether they are a nice trans woman who wouldn't harm a fly, or a violent male predator looking to exploit a societal loophole in order to abuse women.
You cannot allow trans identifying men with gender recognition certificates into women's spaces whilst keeping trans identifying men without gender recognition certificates out. Either you keep them all out, or you let them all in, including the Eddie/Suzie Izzards of this world. Because nobody is standing guard on toilet doors, checking whether they have a magical piece of paper saying they are legally a woman, or whether it's just a case of "on Wednesdays, we wear pink!"
You cannot allow trans identifying men who have had their penises removed into women's spaces whilst keeping trans identifying men who have retained their male genitalia out. Either you keep them all out, or you let them all in, including the Grace Laverys (author of "A Heartbreaking Work of Staggering Penis") of this world. Because nobody is standing on guard on toilet doors, checking whether their penis has been turned inside out, or whether they're still proudly sporting the full meat and two veg.
Which brings me to...
You cannot allow trans identifying men of any kind into women's spaces whilst keeping non trans identifying men out. Either you keep them all out, or you let them all in, including the Levi Bellfields and Vincent Tabaks of this world. Because if all you have to do to gain access to women's spaces is claim to identify as a woman (and, as per the above, it's not practical to use any other criteria), well, any man can claim to be a woman at any time and for any reason.
So the answer is very simple.
We don't allow any of them in.
It doesn't matter how nice or nasty they are, whether they are soft and harmless or a violent convicted criminal, it doesn't matter whether they really believe they identify as women or are just trying it on, whether they almost "pass" or whether they look like a pig in a wig, it doesn't matter how sad they feel not to be included or how uncomfortable they feel in men's toilets.
They are not women, so they can't come in.