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THE SIN OF REMARRIAGE ADULTERY (click here)
The basic idea is that the "exception clause ("saving for the cause of fornication" in Mt. 5:32; "except it be for fornication" in Mt. 19:9) does mot apply to the Gentile world.
Matthew was written by a Hebrew for Hebrews, and the exception clause only applies to the espousal portion of a Jewish marriage prodess, where the premarital year of abstinence before the consummation and the display of the tokens of the bride's virginity proves that the man is marrying a pure woman (Deut. 22:13-21).
That would still be true for orthodox Jews, but not then or now for a Gentile marriage. And you will recall that Matthew (Levi) made mention of this principle in his opening chapter that displays Joseph's concern regarding Mary's status after the Annunciation.
Again, involvement in sexual immorality, which is πορνεία (pornaya, which means kinkiness broader than the limiting translated definition "fornication"). It does include μοιχάω (moichaō, which is straightforward adultery), but it is not to be inyerpreted as a Biblical excuse for divorce in the case when (as in a Gentile marriage) the wedding has been already consummated.
Persistent and unrepentant sexual immorality that damages the plaintiff and/or the rest of the family may well justify separation, perhaps permanent, but divorce? No! That option for God's favor is not open to the offended Christian marriage partner. Jesus' doctrine did not allow for the offended person to divorce the offender, let alone get remarried and taint others.
That stance is not very popular in the Christian community, to say the least, but it is correct, and should be fully endorsed by a person claiming spiritual maturity. I know the cost of proper behavior personally, because I have experienced it for 46 years now, and I am very glad I have taken the path that is just in God's eyes.
Remarriage adultery is a deep sin, as is the acceptance, permission, and promotion of it by Christian leaders or counselors.