Like it or not, actions have consequences. Sometimes, we only think of this statement within the context of our own little world - "My actions affect me." While that statement is true, it's incomplete. In reality, "My actions affect myself AND others." As hard as we may try to escape from the world, 99% of the time, we can't. To complicate matters, it's often hard to predict the ripple-affect our actions are going to have, and sometimes we don't even fully see or realize the full impact of our decisions.
For example, take the conflict going on right now in the Gaza strip between Israel and Hamas. While this not-so-little skirmish is technically going on between only two parties (Israel and Hamas), MANY different parties have been affected. Civilians have lost their lives, the safety of reporters has been put in jeopardy, the attempted relief efforts of the U.N. have been hindered, international relations within the middle east have been strained (even more than normal) - and those are just a few of the ones we can readily see. Imagine how long this list would become if we had infinite knowledge? When one party sins against another, other parties are inevitably affected. Interestingly, the negative affect a divorce has on the children (Party C) is talked about just as much as (if not more than) the negative affect divorce has on the couple (Parties A and B)!
To expand the illustration, think about the inter-connectedness within the Body of Christ, the Church. When one member strays from the Lord, it adversely affects a specific part of the Body, which in turns affects the whole. Truth be told, I am not acting very loving towards my fellow brothers and sisters in Christ when I sin against God. What a humbling thought. I'd like to live as though my sin is my problem, something that I'll deal with in the privacy of my own world. I'd like to live as though my sin is no one's business but my own. What a sad, selfish (not to mention - incorrect) way of thinking. As much as it pains me to admit it, my sin is other people's business. Kind of puts the concept of accountability in a whole new light, doesn't it?