I remember the first time I preached about divorce. That morning one of the members invited her son to hear the new young preacher. He was very recently divorced. Jesus says basically, Don't. It doesn't feel fair. The level of suffering involved in the whole process is so painful. And Jesus says Don't.
Separation is never the answer. Not just in marriage, but period. Jesus whole mission is reconcilation. How can division ever be the right move? There is obviously no sense of hope when the couple decides to divorce. But Jesus says there is. Will it ever be what you hoped? No promise. Paul tells us to stay with an unbeliever in hopes that he will believe.
I've seen hope turn to hopelessness even in staying together. But no hope at all is worse. You have to believe there is something inside the other person, something... You have to hope, and you have to dive for it and bring it back up.