OK, some confusion here.

I started using desktop=gnome because I really didn't like the standard desktop. When you boot with gnome desktop, every available partition is mounted read-write. Maybe that's a bug.

It turns out that this is not the case when you use the default desktop. By default, they are not mounted, but if you mount them in PCMan File Manager, they are mounted RW.

So, clearly I must avoid the gnome desktop and try to live with the default one.

But I would still like to know how to mount a drive read-only via the GUI - I don't really trust myself when it comes to using commands (I've been spoiled, you would probably say).