My husband - an elder in our church - was a model of godliness in public, but the children and I lived in fear of him at home. He had a volatile temper and bullying us was his favorite method of control. If bullying didn't work, he became physically violent - throwing things. He never hit me, but as our son grew into a teenager, he was at the receiving end of my husband's abuse. My husband was also addicted to pornography. I discovered a stash of Playboy magazines about three years into our marriage, and once I learned how to check a computer's browsing history, I realized he spent hours on it.
When I found out that a good friend - a deacon in our church - was having an online affair and wanted to leave her husband, I said the same thing to her that someone said above: happiness isn't the goal, but holiness. I said it out of self-righteousness; at this time my own marital problems were hidden away from the world and seemingly manageable. I think, unconsciously, my attitude was that if I had to stick it out in an unhappy marriage because divorce was wrong, then so should she. I was bitter inside and was too proud to realize it.
Over the next few years, as my husband left the church, quit his job, and became more and more abusive, I learned what real misery is. I often looked back upon my words to that friend and wished so badly that I had allowed myself to truly see her pain and reach out to her in love and compassion instead of smug, thinly veiled condemnation. Of course, sin is sin - but often we ignore the deep, soul-crushing pain that often obscures people's view of Christ and drives them into things like porn and adultery that temporarily relieve the unbearable misery. When I reached out to my friends, they gave the same pat response that I had given the first one - yeah, marriage is tough, but divorce was wrong, and I needed to stick it out and focus on becoming holy. No one wanted to acknowledge or minister to the pain that was wearing me down and eating away at my soul.
Long story short - after 19 years, I divorced my husband. I could no longer justify staying with him "for the sake of the kids", because the effect he had on them was undeniable. They were depressed, withdrawn, and had lost all interest in Christianity. In fact, they had no interest in anything but video games. They too were living in unbearable pain, and trying whatever they could to mitigate it - in their case, gaming. I am so thankful they didn't turn to drugs or alcohol! Our home was not a safe haven for them; I couldn't protect them from his violent temper; and I realized that the example we were setting for them in our marriage could be catastrophic for them, if it was the only example they had.
My now ex-husband went through a tremendous ordeal with his physical health right after our divorce (he nearly died on 3 different occasions), and I have seen a true miracle born out in him. He has been brought to real repentance over his behavior, and has sought to repair his relationship with God, the kids and me. Our children have become bright, engaging, and involved in life once again, and our daughter accepted Jesus as her Savior. Two years ago I hated my ex, and now we have something we never really had - a genuine friendship. Our relationship has a level of trust, honesty, support, and respect that it never had, and I marvel at what God is doing in his life, as well as in my own.
I know some may vehemently disagree with me, but God uses even divorce for good. It brought my ex to a real relationship with and hunger for Him. It gave my children a dad that they no longer feared and hated (yes, they confessed to me that they hated him during those dark years). It healed relationships that I thought were damaged beyond all hope of repair. And it taught me why "love one another" is the greatest commandment after "love the LORD your God". Loving means reaching out to people in pain, even if it's someone having an affair, or a porn addiction, or who can't peel their grasp off the fifth of vodka. It means being steadfast and unwavering in our love, even if we hate what they're doing to themselves and those around them. It means being sensitive to even the faintest cry for help, and being sensitive to the Spirit's direction in how we respond. I hope that the OP and her husband have people who love them like that, and who are willing to be the hands and feet of Jesus in ministering to them in their pain.