Romans 7:1-3 are used by proof texts by legalists to show that there is no right of remarriage. They claim that those verses show that a woman could not remarry as long as the husband was alive. But they ignore that Paul is going by the Law, and in the law, a woman was allowed to remarry after given a bill of divorcement (Deut.24:1-2)
The illustration is that of a divorce, a split, from the flesh and spirit (Col.2:11). This split mean that the Law, which had dominion over us while we in the flesh, no longer has authority over us, just as the wife loosed from her husband in Deut.24 was no longer considered her husband. Our new husband is the Lord Jesus Christ and we are under grace not law.