I may be crazy but I'm pretty sure this option was in a menu at bottom of new Safari app. I pressed this without knowing what it was and now all sites open in desktop version (ie, no m.youtube..., etc.).
As far as I can tell, this menu is gone and I can't turn off the option so the screen view is messed up.
I am seeing this also in iOS 8. Sites that have responsive views are loading those views and then snapping out of it into a full desktop view whether I want it or not and I have not found a way to defeat that behavior. Yours is the first report where it is happening with iOS 7.