7:1 Or do you not know, brothers —for I am speaking to those who know the law—that the law is binding on a person only as long as he lives? 2 For a married woman is bound by law to her husband while he lives, but if her husband dies she is released from the law of marriage.  3 Accordingly, she will be called an adulteress if she lives with another man while her husband is alive. But if her husband dies, she is free from that law, and if she marries another man she is not an adulteress. (Romans 7:1-3 (ESV)
The law is binding on a person only as long as he lives. Paul is building up to the point that we have died. This means the law is no longer binding to us. He brings in the case of a woman bound by law to her husband. As long as he lives she is bound to him. But when he dies, she is no longer bound. Yet in this life she had become one flesh with him.
The topic of marriage in the Bible is much more complicated than is often let on, frankly non of our modern cultural biases, and by modern I include those thought to be traditional, really reflect much of the biblical concept of marriage. What God has joined together let man not separate. Yet, we too often think that joining of man and woman together in marriage is something that God does in a religious ceremony at the church, or perhaps even at the justice of the peace. Frankly, this is a massive misconception. The truth is God joins us together in the carnal act of sex, with or without ceremony. The church, in a sense, has no choice but to view a sexually active pair as married in the eyes of God. They have become one flesh with each other. Paul says this is what happens when a man has sex with a prostitute. So this is true also of high school romances, college dalliances, and after work rendezvous. And that one flesh union remains as long as the two live, or so it seems. The concept of divorce in the Bible is as foreign to us as is the Biblical concepts of marriage, but at least in some case it would seem God did separate what he had joined together while the two remained alive. Here I think of Abraham and Hagar, and numerous other wives Abraham had after the death of Sarah. God also commanded divorce for post exilic Israelites who had taken foreign wives. When Paul in 1 Cor. 7 “allows” for divorce in the case of adultery and abandonment, it would seem that here too has been a separation of that one flesh union to one extent or another, the sin has occurred in that this has been brought about by man rather than God. What is known is that in death God separates a couple. This is apparent from Christ’s answer in regard to Levitical marriage in the temple. In heaven people neither marry nor are given into marriage. They are no longer married, they are no longer one flesh with the other. And the surviving spouse is free to marry again, or stay single. (Paul actually counsels that younger widows should remarry) However, marriage is fundamentally an institution of this world.
Paul uses this earthly institution in many different ways to illustrate the mystery of our union with Christ with whom we become one flesh, one body. He does this most famously in Eph. 5. But it must be said, that though there are many similarities between the realities of our earthly one flesh unions and our embodiment into Christ, they are on another level fundamentally different. For one, we are not joined to Christ in the midst of our earthly lives, but in and through death that we might be joined to him in his resurrection and enjoy the newness of life. This death then becomes our divorce from sin, but also from the law. It is in the death of Christ that God separates what he had previously joined together, our fleshly union with sin and the law. And since it happens that Christ has been raised from the dead to live for all eternity, God never separates us from him, there is no expiration date on our union to Christ.