> So why would people care if there were women there? Is it different?

Yes, it's way different, obviously!

> I’ll go a step further and say I think the entire concept of a single-sex changing room may be outdated.

You're definitely in the minority there. Having been a 13 year old boy myself, and knowing how 13 year old boys behave, there's no way in hell I'd allow a room full of them to be in a room with my 13 year old daughter naked.