The larger context of this passage involves Paul's admonition not to engage in activities you do not beleive to be sinful if you know that someone else does believe the activity is sinful and also have reason to believe that your actions might cause that person to follow your lead. In those circumstances, you might not hve done anything wrong per se, but you would have contributed to someone else violating their conscience.
That is not what I want to talk about now. Instead, I want to talk about why it's so bad to violate your consceience, or even just to do something you find morally questionable.
Paul makes no secret, especially in Romans, of the concept that everyone has within themselves sufficient knowlege to discern right and wrong. That moral sense may not be as refined as what you would find in the Torah, but it is still there. We typically call that your conscience and Paul referst to people having the, "law written on their hearts."
As I said, our conscience is not as refined as the Torah or other explicit definitions of God's law. Consequently, it would not be fair to say that if you don't feel something is wrong, then it's OK. It would be accurate to say that if you thinik something is sinful, then for you it is.
Paul actually takes this one step further in Chapter 14. He tells us that everything we do is to be an act of faith in God. Therefore any act we commit which we are not sure, through our faith in Christ, is what God would have us to do, is sin. This is not because the action was wrong per se, but because the motivation was wrong.
Any act, no matter how noble, not performed is an act of faith is sin. This has two implications. The first is that there are no grey areas. If we are not sure we are doing the right thing, then we aren't. The first is more far reaching: In God's eyes morality is not just a set of rules, it is the expression of our relationship with God. Seen in that light it becomes obvious that obeying the law is the minimum, not the maximum, requirement for avoiding sin.
Truly living a sin-free life requires a total reordering of our lives and our priorities. I wonder if we really take that as seriously as we should. I know I often don't.