I don't like sharing toilets with men - I really, really, hate unisex bathrooms. It's not because I think men are going to attack me, it's because I feel vulnerable bleeding. If you tear open a pad in the women's toilets, no one is going to care or judge you, because everyone in the women's toilets knows that sound is a normal sound.
If you're having some kind of malfunction, another woman will help you and keep her mouth shut because there's a girl code. If you've changed outfit and/or are styling your hair/makeup because there's a special event you're going to, another woman will tell you that you look great if you're having a wobble, and it won't be a sexualised comment.
If you're sobbing your little heart out because you just got the worst news of your life and want support (indicated by crying by the sink rather than in a cubicle), a woman will give you a hug, even if you've never spoken before and/or you hate each other normally.
Women's toilets are more than just places you go to urinate and defecate. They're places where we look after ourselves and each other.
When it comes to colleagues, you tend to know more about them than you should. If a colleague was transitioning from MTF, I'd probably know that she was genuinely taking steps to become a woman. It therefore wouldn't bother me.
I take issue with blokes taking the piss and randomly 'identifying' as a woman to get into our safe spaces without actually meaning it. Just saying you're a woman doesn't make you a woman. You have to either be biologically a woman and tick the box by default, or to mentally feel like a woman and as such, really mean that statement.
If someone is so unhappy with their birth sex that they are very obviously changing their appearance, then I will happily let them into the sanctuary of the women's toilets even though they don't have exactly the same hardships. They share enough of them to be welcomed in.