We spend a lot of time searching for holes in the fencing of our playground. Under what circumstances am I allowed to approach this or that forbidden thing. Divorce is certainly one of those things and, although in today's world it is comfortably assimilated into most cultures, it always has been.
The religious leaders tried to entrap Jesus on this very question. The oft quoted 'adultery exemption' is weightier in the whole context.
And Pharisees came up to him and tested him by asking, “Is it lawful to divorce one's wife for any cause?” He answered, “Have you not read that he who created them from the beginning made them male and female, and said, ‘Therefore a man shall leave his father and his mother and hold fast to his wife, and the two shall become one flesh’? So they are no longer two but one flesh. What therefore God has joined together, let not man separate.” They said to him, “Why then did Moses command one to give a certificate of divorce and to send her away?” He said to them, “Because of 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 sexual immorality, and marries another, commits adultery.” - Matthew 19:3-9
What Jesus outlines here is the difference between what God intends and what God allows because we are hard-hearted. Obstinate, unrepentant pursuit of sexual sin within a marriage is allowed by God as grounds for divorce but let's not deceive ourselves into thinking it pleases Him. Divorce is an act which tries to unmake a creation of God: "So they are no longer two but one flesh. What therefore God has joined together, let not man separate."
We err greatly in the matter when we hold that a secular court or an ecclesiastic body either enacts or dissolves a marriage. "What therefore God has joined together, let not man separate". This is a divine, creative covenant established directly by God before the intrusion of sin into creation.
The book of Job weighs in here without direct reference. When Satan is given permission by God to oppress Job,
“Behold, all that he has is in your hand. Only against him do not stretch out your hand.” - Job 1:12
Satan is forbidden to touch Job himself. Satan destroys Job's propsperity and his posterity but, although Job's wife is never mentioned in the prohibition, Satan leaves her alone.
Again, when Satan is allowed a second time to afflict Job,
And the LORD said to Satan, “Behold, he is in your hand; only spare his life. - Job 2:6
he obeys God's prohibition and does not take Job's life. Once again there is no mention of Job's wife in the prohibition but Satan leaves her alone.
It seems that in the heavenly realms it is understood as a matter of course needing no mention (not even to the devil himself) that a man and his wife are one and the same entity.
And this second thing you do. You cover the LORD's altar with tears, with weeping and groaning because he no longer regards the offering or accepts it with favor from your hand. But you say, “Why does he not?” Because the LORD was witness between you and the wife of your youth, to whom you have been faithless, though she is your companion and your wife by covenant. Did he not make them one, with a portion of the Spirit in their union? And what was the one God seeking? Godly offspring. So guard yourselves in your spirit, and let none of you be faithless to the wife of your youth. “For the man who does not love his wife but divorces her, says the LORD, the God of Israel, covers his garment with violence, says the LORD of hosts. So guard yourselves in your spirit, and do not be faithless.” - Malachi 2:13-16
Is marriage extremely difficult and painful at times? What part of a life of faith is not?