Paul was an authority on the Pentateuch and understood how God created man. As an apostle he stated God's command of "to the married I give instructions, not I, but the Lord, that the wife should not leave her husband" (1Co 7:10). However, "if while her husband is living she is joined to another man, she shall be called an adulteress" (Ro 7:3). This is reinforced by Jesus saying adultery is committed (Mt 5:32). But Paul says "if you marry you have not sinned" (1Co 7:28). Jesus' and Paul's teachings have to agree.

What are the ground rules? "The married woman is bound by law to her husband" (Ro 7:2) and is "bound as long as her husband lives" (1Co 7:39). "He cannot divorce her all his days" (Dt 22:29). But she would be able to remarry "if her husband dies [because] she is released from the law concerning her husband" (Ro 7:2). There are principles as with laws of nature. Even though Moses once permitted divorce, you couldn't return to your previous husband once you remarried (Dt 24:4). It's like once they "have fallen away, it is impossible to renew them again to repentance" (Heb 6:6).