Perception/vision is not infallible. Sometimes we can be looking right at something and not notice it or immediately forget what we saw (I remember a TV documentary a few years ago that showed how unreliable eye witness testimonies are, because of this).
In normal, everyday scenarios this is the case. Now thrown in a frantic, high-pressured environment like, say, refereeing a PL match. Yes, it appears to us that Marriner was looking right at it and therefore should have 'seen' it; but he didn't, and it's totally understandable why.
Yes, Aguero was reacting to being fouled by Reid. Yes, Fellaini and Costa are consistent offenders. But Aguero can't control those things, he can only control what he does. He reacted, it was stupid and it was no-one else's fault but his own. He got away with a 'stamp' on David Luiz a few years ago so I guess his luck has run out. We will all just have to suck it up and focus on beating Utd with the other 15 or so excellent players that we have..