I do think selfies in the loos are quite a big thing with young women.
Because they're often dressed up on a night out, have had drinks, are in there with friends, there are mirrors to pose and touch up, and it's a separation space/ break from the mixed sex social space.
I don't do this myself as I'm a middle aged woman so a quick hand wash and checking my skirt isn't tucked into my tights suffices, but it's silly to argue girls don't do this.
Girls have always bonded over dressing up, playing with beauty and fashion and doing this separately from the boys. The loo selfies are a modern extension of that.
This is why the TWs want in on the women's loos for the full 'girlie' experience.
I don't think it's helpful to say there is no female hanging out and bonding going on in loos, which now might involve selfies, but instead should argue those girls have a right to have a space for their antics separate from men just because they want to.