The following is an excerpt from The MacArthur New Testament Commentary on Romans 7.

For the married woman is bound by law to her husband while he is living; but if her husband dies, she is released from the law concerning the husband. So then if, while her husband is living, she is joined to another man, she shall be called an adulteress; but if her husband dies, she is free from the law, so that she is not an adulteress, though she is joined to another man. (7:2–3)

Contrary to the confusing interpretations of some commentators, the apostle is not presenting a complex allegory, or an allegory of any kind. He is simply making an analogy to marriage law to illustrate the single point he has just mentioned, namely, that no law has jurisdiction over a person after he is dead. This passage has absolutely nothing to say about divorce and cannot legitimately be used as an argument from silence to teach that divorce is never justified for a Christian and, consequently, that only the death of a spouse gives the right to remarry. (Such a discussion requires treatment of other passages, such Matt. 5:31–32; 19:3–12; and 1 Cor. 7:10–15. For further study see the author’s book The Family [Chicago: Moody, 1982].)

Paul is calling attention to the fact that marriage laws are binding only as long as both partners are alive. Being joined to another man while her husband is alive makes a woman an adulteress, an offender against the law. But to be joined in marriage to another man after her husband dies is perfectly legal and acceptable. A widow is absolutely free from the law that bound her to her former husband. Paul, in fact, encouraged young widows to remarry. As long as they were joined to a believer (see 1 Cor. 7:39), such widows, he says, should “get married, bear children, keep house, and give the enemy no occasion for reproach” (1 Tim. 5:14).