I'd say that you're mixing two different things; on the one hand, sin, while on the other, public morals/ethics.
For sin, there is no fudging; anything that falls short of perfect righteousness is sin, period -- there's no grade scale, no grading on a curve, there's just what an engineer would call 100% inspection and 100% compliance. It doesn't matter if an action is meditated and carried through, is done on impulse, is done without even thinking about it, or even is done without being aware of the action; all that matters is that it is not 100% compliant. This is what Jesus was pointing to when in the Sermon on the Mount He told the crowd, "You have heard it said.... but I say....", moving the "goalpost" from outward physical action to just the thought regardless of whether the thought is carried through. And His conclusion of that discourse was, "So be perfect/complete as My Father in Heaven is perfect/complete". If an action, or even a thought, isn't perfect/complete, it's sin -- and in fact the Apostle takes this theme to its ultimate conclusion, that "everything which is not of faith is sin".
But in society the concern isn't sin, it's public order and safety. Public order depends on what a society considers good and what it considers bad, not on any absolute scale, and thus public morals are by nature somewhat situational and also somewhat flexible -- and thus situational ethics can come into play, depending on the basic moral concepts of the society; a society where morals are defined according to standards of sin will be far less flexible, which means far less understanding, compassion, and mercy, while a society that is based on some shared understanding of what is good and what is bad will have much more flexibility and so have far more room for understanding, compassion, and mercy. This also means that whether a given system is objective can only judged by how much that society understands what is and what isn't actually harmful to others, and whatever standard is set will also bend according to what a society considers heroic or worthy of praise.
Ironically this means that for a society to be stable it cannot hold tight to rigid standards, i.e. to measures of sin, because that society will be less capable of compassion and mercy, while at the other end of the spectrum a society that is too flexible will also not be stable because there will be no agreed-on measure of justice, let alone its positive companion virtues.
Since your question looks at associating with other people, then you have to find a balance that works for you -- a truth that requires you to be flexible and thus to some extent view things relatively. How do you decide on your public standards, then? St. Paul offers some advice: insofar as is possible with you, be at peace with all men. In more modern terms we might say, as much as you're capable, get along with people.
