My mistake I thought you were referring to the Secondary Menu.
But this is actually how all Genesis themes function when using Sub Menus that expand beyond the width of the mobile browser. I still wouldn’t consider this a bug since all you would have to do is open up the menu, and then drag/slide the site over to see the submenu. I believe this is the intention with the Genesis Framework. You can see the in Genesis Demo – http://genesisframework.com that the same thing occurs.
Any other time the site can’t be dragged over to the left or right since it’s responsive and will already be adjusted to fit your screen, but once you open a submenu your phone’s browser will allow you to move the site to the left in order to see whats on the right. Does that make sense?
I just checked your site on my phone and it looks fine, even with the submenu expanding beyond the screen. I’m sure most of your visitors will automatically know to drag the site over when they see a menu expand.