I am confused about your definition of porneia in Mathew 5:32 and Matthew 19:9. In your answer to the question "My wife is leaving me after only seven months of marriage. Where does this leave me?" you said that: "One of the reasons for the divorce has to be because the other person is committing sexual sins. Jesus isn't talking about later, after the divorce was finished." By that I think you mean that if a person divorces his or her marriage partner, and then commits remarriage adultery, the first marriage is still valid, and it is clear that the first marriage is still valid because, otherwise, the second marriage could not be adultery. So divorce followed by adultery does not dissolve the first marriage, and such adultery does not dissolve a marriage even though a divorce has occurred. Because the first marriage is still valid the innocent party cannot get remarried. However you say that adultery followed by divorce does dissolve the first marriage, and the innocent party, but not so innocent as in the previous case because he or she has decided to divorce, can get remarried. However if you leave it too late and the guilty partner divorces you first, because he or she wants to get remarried, your marriage to that guilty partner is still valid. Is that truly what the Scripture is saying?
Perhaps your definition of porneia is incorrect, and it refers to something that has occurred before the marriage ceremony, not that has occurred after. The "exception clause" is in Matthew only, not in Mark and Luke, and there is an opinion that Matthew was written from the Jewish perspective, and perhaps therefore the "exception clause" applies to the Israelite marriage system, where a betrothal was counted as a marriage, even though the marriage ceremony had not yet taken place. Mary was betrothed to Joseph. Joseph decided to divorce Mary, before the marriage ceremony had taken place, because he thought that she had been unfaithful. Perhaps therefore the "exception clause" refers to virginity fraud. I think that God divorcing Israel probably fits into this category. Please confirm this, if you think that is true. After the marriage ceremony has taken place, Scriptures confirm that nothing but death can end a marriage. The permanency of marriage is even used as an example in Romans 7, and if marriage is not permanent, ending only at death, then how could it be used as an example? Mathew 5:32 and Matthew 19:9 (and Mark 10:11-12 and Luke 16:18) are saying, I think, that not even adultery can end a marriage (after the wedding ceremony has taken place), not that it can.
That the Bible states that divorce can end a marriage is seen in: "But to the married I give instructions, not I, but the Lord, that the wife should not leave her husband (but if she does leave, she must remain unmarried, or else be reconciled to her husband), and that the husband should not divorce his wife" (I Corinthians 7:10-11). The use of the word "unmarried" shows that the marriage ended, yet the rule is that ending of a marriage does not give the individuals permission to marry another person. Their covenant remains in effect.
A marriage to someone else while still bound by the covenant is adultery. "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" (Romans 7:2-3). "Law" here is being used as a synonym for covenant (see Psalms 78:10 as evidence that law and covenant can refer to the same thing).
Jesus, by implication, gave one exception that allows one party in a covenant to be released prior to the death of the other party and that is due to fornication on the other person's part. "And I say to you, whoever divorces his wife, except for sexual immorality, and marries another, commits adultery; and whoever marries her who is divorced commits adultery" (Matthew 19:9). There are two conditions that are being stated: a divorce not due to fornication on the other person's part, and a remarriage. When those two conditions are met, there is adultery.
In Matthew 19:9, there is no time element in regard to when the fornication took place -- neither in the English translation or in the Greek. To impose a time element to add to the passage. All that is stated is that fornication took place or is taking place.
That the Greek word porneia can include adultery is seen in I Corinthians 5:1, where a man was having sex with his father's wife. Such is would be adultery (Romans 7:2-3), but it is also fornication because fornication can be used generically to cover all illicit sexual acts with another person. For a detailed definition of porneia, see: Scripture Notes on Sex.
While a betrothal was seriously considered, it was not a marriage. You will find that Joseph took Mary as his wife after the angel assured him that the pregnancy was due to God (Matthew 1:24). If Joseph had ended his engagement to Mary prior to their marriage, there would no restriction on either one to marry someone else. After all Gomer, an immoral woman, married Hosea; therefore, sex before marriage is not a cause for a marriage not to take place or to continue.
I'm sorry but your understanding of Matthew 5:32 and Matthew 19:9 do not match what the passages actually state.