I think you are right to a point, people's judgement will always be to a certain extent subjective, but I disagree with you ultimately for two reasons.
first I think we can and often do agree on what amounts to sensible or reasonable behaviour. I work in the criminal justice system and we routinely ask juries (a microcosm of society) to consider what is or isn't reasonable behaviour in a given context. and juries are in my experience often good at expressing clear and consistent collective ideas of reasonableness. the concept of un / reasonable behaviour features in a lot in our criminal law, which in itself is simply an expression of a common or joint sense / morality.
secondly, to reject utterly the idea of common sense seems a very dangerous and unattractive thing to do. what does it mean for society? that we have no shared concept of what is sensible, reasonable behaviour? that we are all entirely subjective and relativistic in our outlooks? hyper-individualism and the disintegration of social bonds is a real issue in my view for society...people will always of course disagree, a debate is as much a part of society as anything else. but fundamentally we have to believe that there is more that unites us than divides us, otherwise what are we left with?