Gender segregated bathrooms were invented and designed to protect women from men. It is one of those "Patriarchal" institutions that Feminists don't seem to really mind that much, and instead of abolishing it, they'd rather come up with a third and a fourth room, extra stalls and whatnot.
Frankly I'm for abolishing gender segregated bathrooms. We already have laws covering everything from harassment to rape. So we hardly need to separate bathrooms to "protect" women.
But again, it's not about what is practical, but about how women "feel". So in order to "feel" safer, they'd rather spend hours queuing for the loo at concerts, parties, restaurants, night clubs, public events etc.