I think that there is a large misconception about the safety of women's restrooms and fitting rooms. Many women think the women's room will be a safe place for them when, in fact, it's not. There is more abuse against women in the women's room than anyone would imagine. Also, the more bystanders, the less likely that anyone will step up and do something about it. So, letting the transgendered use this so-called "sacred space" isn't going to make any more of a difference than if they're not allowed to use it.
Women are a heck of a lot more capable of fighting back then they even think they are. I can't tell you how many people I've scared off without even committing assault! And I'm probably one of the least scary looking people you'll meet (until I summon my Women's Wrath technique! >

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Anyways, I've said it once and I'll say it again: The best solution for this would be the avaiability of single-use gender neutral restrooms. Not TRANSGENDER restrooms, because who in their right mind would out themselves like that?! But I myself would use a unisex restroom if it's single use, because I like the extra privacy (and it helps if you have a shy bladder!) If somebody doesn't pass or just doesn't feel comfortable choosing a gender-specific facility, well, they can use that room and not have unwanted attention for it.
When the "family bathroom" is unavailable, I usually accompany Jocelyn to women's room. It's seems like if other women see me allowing it, they're okay with it. And I usually act as "look out" for getting her into a stall unnoticed and letting her know when the coast is clear to come out of the stall. We do try to avoid needing the bathroom, but it's not always that easy. She tries not to drink anything so that she won't have to go, but then if I have to go, I bring her with me because I won't leave her alone in public (and she doesn't want to be left alone in public). So, it's a bit of work but nothing really bad has happened so far.