Verses 8-9 : for singles and widows
Verses 15, 27, 28A : for believers with unsaved spouse; allowed to re-marry if separated
Verse 39 : for saved couples; both believers; NOT allowed to re-marry if separated
1 Corinthians 7 is in one way or another related to Romans 7 as it also talks about ,marriage but only with saved couples. It shows about the allegory of Christ and His assembly which is indissoluble, eternal and can not be broken. It is a relationship that is bound together, oneness, like the body [Assembly] and the Head [Christ]. The chapter also defines the responsibility of the Christian couple to keep their relationship intact as they live through and against the world of sinners (Romans 7:3, 1 Corinthians 7:39).
Often times, Christians mix 1 Corinthians 7 teaching to Matthew 5:31-32. But keep in mind that Matthew is addressed to the Jewish people and not to the Assembly.
To understand the marriage instruction for believers (the Assembly), forget about Matthew 5. Never mind the old Jewish ways of marriage and stop digging old bones. Instead, FOCUS on 1 Corinthians 7 which contains the teaching and the "rule" (verse 17B) given for all believers to follow. It is very easy to understand. You will not be confused.
After meditating on 1 Corinthians 7, notice that every angle, from single to married people, is mentioned. And the first verse of the chapter clearly indicates that the motive of this chapter was to clarify the confusing aspects of marriage. The Corinthians wrote Paul questions regarding marriage and Paul, in this chapter, answered in detail what to do. It is a complete chapter in a sense that you don't have to go anywhere else to understand. It only took forty verses to contain everything we, Christians, need to know about marriage.
1 Corinthians 7:15 "But assuming that the unbelieving husband departs let him be departing. A brother or sister is not in the position of a slave, namely, bound to the unbelieving husband or unbelieving wife in an indissoluble union in case such as these;..."
1 Corinthians 7:27-28A "Have you been bound to a wife? Stop seeking to be loosed [divorced]. Have you been loosed from a wife? Stop seeking a wife. But and if you marry [re-marry] you did not sin."
These verses refer to a married couple consisting of one believer and an unbeliever [unsaved]. In the case the unbeliever decides to leave and the believer finds himself divorced (not bound anymore) and decides to marry again later, verse 28A says he did not sin to find another wife. But this time, he should be married with a believer.
So much has been said on the subject of marriage. We acknowledge that the teaching presented above will stir up disagreement and others would say it is too simple to believe. But I do believe that the answer of the Assembly for any question regarding marriage (and even other concerns) should be found in the teaching of the New Covenant Assembly. A lot of believers venture out in the First Covenant to try to understand the position of the Lord Jesus related to the Assembly, but this only brings more confusion because the First Covenant does not belong to the Assembly. The teachings of the Assembly did not come out from the First Covenant but from a direct revelation from the Lord after the Holy Spirit came down (Acts 2).
We can not pick and choose just whatever we want from the First Covenant and try to make a doctrine out of it while ignoring other First Covenant teachings such as that of a woman caught committing adultery should be stoned to death. The New Covenant does not teach that. So if one believer takes a teaching from the First, he should take everything in it including the stoning of a woman to death, which obviously would make no sense. If a Christian today does not make a difference between the two, he is in a great danger of falling under the law. "Because the law through the intermediate agency of Moses was given, the aforementioned grace and the truth came through Jesus Christ" (John 1:17).