The Apostle Paul used marriage as an illustration of the binding power of any law - to introduce his teaching about the powerful effect of the 'law of sin' (Romans 7:4-25).  It was not his primary intention here to preach about marriage, and certainly not to suggest that marriage is in any way sinful.  However, God has used these verses to remind Christians of every generation of His intention that marriage should be a binding, exclusive, bilateral covenant which lasts for life.

It is both God’s law and an internal law to the human soul.  To flout that law is to dishonour God, dishonour one’s spouse and wound the perpetrator as well as the victims (1 Corinthians 6:18).  Jewish believers in Rome understood this principle well from the Old Testament law, and Gentiles would have known the personal and social pain which always occurs when the sanctity of marriage is spoiled.  

God intends that a husband and wife should be bound to each other until one of them dies (Genesis 2:24, Matthew 19:5, Mark 10:7-8, Ephesians 5:31).  After that the surviving spouse is ‘released from that law’ and is not married any longer.  Although the person will be mourned and the memory of the relationship will be treasured, it is not wrong to marry again.  But before then, any other sexual relationship is adulterous.

In a fallen world where everybody is infected with sin and easily swallow Satan's lies of temptation, every relationship will fail to live up to God's perfect design.  So, it is important for married people to guard their bonded relationship and, whatever the difficulties, to make the very best of the precious gift they have been given.  That is not just because a good marriage is better than a bad one, but because there is a God-ordained obligation which binds husband and wife to each other.  Alas, sometimes things go wrong: when wrong decisions have been made, the only doorway into God's healing grace is repentance.  Where restoration is possible, it must be humbly pursued; where not, only God's grace can sustain sinners.