"You shall not commit adultery." Exodus 20.14
The 7th Commandment exists to protect the marriage bond, which provides the healthiest context for enjoying intimacy and rearing a family. As such, marriage is the bedrock of society and human happiness. To threaten such equilibrium is a sin - and not just against those involved. The Bible is clear that all sin is ultimately offensive to God, who makes the rules - for the flourishing of his creation. It is a sobering thought that, every time we contravene these laws, we are not only injuring our fellow creatures but also our Creator. To realise this is to begin to get our heads around why sin is such a big deal. However trivial or serious one sin might appear in relation to another, the fact is that it is all inter-connected and it all contributes to the breakdown of that web of relationships binding humans and the natural order under God - to the detriment of us all.
"But only Jesus lived a sinless life. It's too much to ask the same of us lesser mortals!" I hear you cry. This is true. Yet a realistic appreciation of the awfulness of sin at least illuminates why it is such a problem, how desperately we need a Saviour and the graciousness of God in coming in the flesh and squaring the circle: his righteousness for our sinfulness, through the death and resurrection of Jesus. That work of redemption might continue to be a work in progress for now but there is reassurance in knowing it is underway and it gives those who believe an answer to the problem of sin that would otherwise be lacking. Just as importantly, acknowledging our plight and perceiving how we are rescued renders us a people of hope rather than of despair.