He doesn’t say whoever presumably marries or tries to marry. He says marries. He doesn’t say presumes to marry or tries to marry—as if, yes, this is a real marriage being created. It should not be created and it is like committing adultery when you enter it. He says a similar kind of thing in Mark 10:11–12. He said to them, “Whoever divorces his wife and marries another commits adultery against her, and if she divorces her husband and marries another, she commits adultery.”
So, if Jesus is willing to call wrongfully entered relationships marriages, then it seems to me that we should hold people to the expectations of holiness and permanence implied in the word marriage, till death do us part. I take the warning that remarriage involves adultery, “Whoever divorces his wife and marries another commits adultery,” not to mean that sexual relations in a wrongfully entered relationship can never be sanctified through repentance and forgiveness, but rather that an unholy relationship involves unholy sex until that relationship is newly consecrated to God through repentance and forgiveness. That relationship remains tainted at every level.
4) One last thought on divorce and remarriage. If this seems strange that a prohibited relationship can become a consecrated and holy one, consider the example—and there are several in the Bible—of the kingship of Israel. The people came to Samuel in 1 Samuel 8:6–7 and said, “‘Now appoint for us a king to judge us like all the nations.’ But the thing displeased Samuel when they said, ‘Give us a king to judge us.’ And Samuel prayed to the Lord. And the Lord said, ‘Obey the voice of the people in all that they say to you, for they have not rejected you, but they have rejected me from being king over them.’” And yet, in spite of this evil origin of this new relationship of king and people and God, God made the kingship an integral part of his plan for Jesus to come as the King of kings and Lord of lords and as the Son of David.
So, for those four reasons, examples, kinds of texts in the Scripture, I would appeal to those in a marriage they should not have entered to remain there and consecrate themselves by confession and repentance and consecration so that henceforth they will keep their sacred promises.