Ever since the outcry when DeBruyne was awarded the pen against Swansea we don't get penalties anymore unless they're completely obvious.
That said, how Clattenburg missed that one I've no idea, surely he saw Sterling get the touch before Robles? Surely he also saw Robles steaming out of his goal? Even if there's no contact, which their clearly was, then Sterling has to take evasive action to avoid being clattered, that's a foul Mark.
And before any starts, I'm discussing an incident in the game, not blaming the ref for the result.