I didn't want to write this... But here is the bummer news...
Do you know why an iPhone 5 with a case ALSO bends? The sad news is that in order for a case to be tight and snuggly with no air gaps, it must wrap the phone and apply pressure on it from all sides. Unfortunately, you can't cover the front glass (because you wouldn't be able to use the touch screen). So all cases usually cover the back and the sides of the iPhone 5 tightly. It needs to be tight because if not, the case will fall off (especially the rubber types).
Now, think about that a moment. Why does the phone bend backwards (the corner and sides) away from the glass side and the bent part is the aluminum near the volume button hole? Why doesn't it bend away (the corner and sides) towards the glass? Because the glass is rigid and will push back, and can withstand bends better. But the thin aluminum back is weak and will bend at the lightest pressure (near volume button). So if you have pressure, the bending away (corners and sides) will go towards the side that is weakest... the aluminum back side. It also doesn't help that all cases pull the iPhone 5's sides towards the back (there is no case cover on the front of the glass applying pressure). The part that will give to the bend first is the volume button hole. The best analogy is if you were to hold the iPad mini sized cardboard with one hand and then you squeeze hard. The cardboard middle will bend away from your hand (just like the iPhone 5 side and corners will bend away from the glass front). Your hand is the iPhone 5 case, the weak cardboard is the iPhone 5.
So everyone that thought you could buy a case to make your iPhone 5 more rigid is actually weakening it by bending it. Of course if you sit on it, doesn't matter what side, it will bend that side first because the glass will return to original shape, but not the aluminum. So it is a tradeoff not getting a case (dropped/scratched iPhone 5) or gettings a case (bent iPhone 5).
I don't know how this can be fixed, besides tempered aluminum of a higher grade.