After comparing the Christian life to slavery, Paul used another relationship to illustrate the change from the Law of Moses to grace. He reminded the Romans that in marriage, a husband and wife are bound to remain together until one of them dies. After that death, the survivor is legally dead to that relationship and is free to remarry.
God’s people were married to the Law of Moses until the death of Christ on the cross. At that time, the law was taken away (died). Man then became dead to the law and was free to remarry spiritually. One becomes married to Christ when he obeys the gospel, becoming a Christian.
Some may have asked if the law was sin since Paul had so vigorously taught that it had been superseded but the law had a purpose as he had stated before. It made man aware of sin by identifying it and by serving the Jews until the law of Christ would come into effect.