Paul is continuing to answer the question of whether or not because we are justified by grace and not by works, whether then how we live actually atters? Can I not just continue to sin? If I am dead to the law, can I not live however I please? Surely I have no need to keep the law?
Paul has previously used the analogy of being slaves to sin and slaves to God and now he turns to marriage. We are as such ‘married’ to Jesus and no longer married to sin. Therefore our desire is to please him. So we have a new motivation to keep the law out of our delight and joy, rather than as a demand.