Again: what is this alleged exception, exactly? You're allowed to divorce and remarry if your spouse commits adultery? What about your own adultery? What about sexual infidelity that stops short of adultery? What about non-sexual infidelity? Where does this stop, exactly? GotQuestions shows the problems with this view. Eslewhere, it claims all sorts of exceptions to the prohibition against divorce:
If you take porneia in the broad sense, allowing remarriage where there's been sexual immorality in the marriage, it would blow a huge hole into the permanence of marriage. After all, if marriages are permanent unless one partner commits a sexual sin, how many marriages would survive? The exception would quickly swallow the rule. The people claiming that Jesus is creating an exception don't seem to have a clear understanding of what this understanding is, or what the limits of it (if any) are.
This isn't idle speculation, either: Evangelicals are among the most likely to divorce (even worse than non-religious Americans). This marriage crisis isn't assisted by preachers claiming that divorce and remarriage are okay in the (all too frequent) cases in which one or both partners have committed sexual sins.
For those who claim that Jesus is creating an exception for adultery (or for sexual infidelity or immorality), consider the Biblical context. The Pharisees asked, “Is it lawful to divorce one’s wife for any cause?” (Mt. 19:3).
This question arose out of a dispute between two Jewish schools. Deuteronomy 24:1 permitted a man to divorce his wife if “he has found some indecency in her.” Was this indecency referring to adultery, or any fault? The great rabbi Hillel the Elder had claimed that it permitted divorce for any indecency, opening the door to divorces over completely trivial matters. More conservative rabbis claimed that the indecency in question was adultery. So those were the two camps (and basically, the Protestant positions today).
If Jesus was saying that there's an exception for divorce, all He had to do is say that the conservative camp was right. But He doesn't. Jesus rejects both camps, saying (Mt. 19:4-9):
They said to him, “ Why then did Moses command one to give a certificate of divorce, and to put her away?” He said to them, “ For your hardness of heart Moses allowed you to divorce your wives, but from the beginning it was not so. And I say to you: whoever divorces his wife, except for unchastity, and marries another, commits adultery; and he who marries a divorced woman, commits adultery."
So Jesus calls us back to the indissolubility of marriage, the Pharisees invoke Deuteronomy 24:1, and Jesus revokes the adultery exception to restore marriage to its state prior to the Law.
And today we celebrate...
