It depends on which kind of airplane. It also depends on the altitude and whether there is cabin pressurization. There ARE a number of emergency exits on large airliners which can quickly be opened when on the ground...or on the water, in the case of ditching the plane. Those suffice. It wouldn't be technically practical or sensible to make all the windows on those large planes into emergency exits.

Large planes also have oxygen masks which drop down in the case of an emergency, and that covers the problem Romney is alluding to. It seems quite odd that he would talk about opening the windows on such a plane.