A sin is anything that is opposition to God's laws. If you believe that your behavior is always taking a stand either for GOOD or against right, then your choices and their consequences are either for good or not for good - not necessarily evil.
Basically the way I see it - there is no 'fence sitting' with God. You are either progressing towards Him or you are digressing away from Him. That's often why christianity often appears to the on-looker as being so Black & White. No man can serve two masters - the bible tells us. And I believe that to be so. Light & Darkness, Black & White, one side of the line or the other. That would be the ultimate in hypocracy if we were attempting to walk both sides of this 'line' - wouldn't it? Just because something doesn't APPEAR to do harm - doesn't mean it doesn't have eternal consequences either for ourselves, others, our spirits, our future, future generations etc - the list is endless of how & in what way our choices affect others & other things. Those consequences are most often what determines the degree something is or is not sinful.
There is a story I like about a man looking to hire a stage-coach driver. Great drivers came to try out for the job on a windey, dangerous, cliff ridden mountain road. Most would drive as fast and hard and as CLOSE to the edge of the cliffs as they could to try & impress the owner. In short he ended up hiring the driver who drove as far AWAY from the cliffs edge as he possibly could. We likewise should try to live like this safe man - keeping as far AWAY from the cliffs that Satan puts in front of us where in lies safety and a life as far from sin as possible..... Our children at church sing this really great little song who's words I really like...and the older & wiser I get the more I understand their significance - it goes like this "Keep the commandments - in this there is safety & peace."