It is impossible for all divorces to be automatically sinful because God has been through divorce. That's right, check it out in Jeremiah 3:8 "Lo I had put her out and given her a certificate of divorce"...He is speaking of Israel.
Whether you consider this a literal divorce or not, if ALL divorces were a sin, you can bet those words would never have escaped Him. There is no place in the Bible where God Himself states, "Lo I committed adultery with her", or "Lo I lied to her", or "Lo I cheated her". That's because those things are sin - all the time.
Both marriage and divorce can be sinful depending on the circumstances. In an absolutely perfect world, the union between a man and a woman would be perfectly initiated by God and there would not be any destructive forces tearing at the bond. Marriages would never break down.
But this isn't a perfect world and some marriages become so emotionally, spiritually, and/or physically destructive that divorce can be life saving. That is why it is dangerous to slap a grid of legalism onto yourself or others.
The Bible does not forbid divorce, but God calls us to examine our hearts and to follow His specific leading. God wants us to live abundantly in His blessings. He will not entrap us, or force us to stay in our own traps to uphold an institution. The institutions of our loving God are never more important than the people within them. If He directs you to restore your marriage, then He has in mind a complete restoration of your relationship so that you can grow together and even exceed the love you had when you first met. Most marriages that fail do so for silly reasons when an attempt to reconcile would bring healing, health, joy, and strength.
God's idea of restoration is not just that you stay in the same house miserable so that things look good on the outside! God's idea of a marriage is an active thing where both people support, build, cherish, and nourish each other in Him. In fact, where exactly does the Bible say that marriage is an institution? Hmmm. If I had a choice between institutional food and a home cooked meal, I think I'd choose option B. I would rather think of a marriage as an ongoing love relationship that is so deep that many ministries and much abundance springs from it.
Galations states that we are no longer under law, but under grace. It is monumentally unhealthy to contend for a marriage on the basis that it is the legalistic and religiously correct thing to do. The greatest reason to contend for a marriage is that it is a love relationship. What we want is not so much to keep a marriage intact to outside appearances, but to genuinely heal the love. Then, we want to guard against that love being attacked and torn down again.
Churches should not be controlling people's decisions about marriage and divorce. Leaders should not be enforcing heavy loads of false doctrine that they have no part in carrying. As a Body, we should support our close friends and help each other make honest decisions. In troubled times, it's a good idea to get feedback from those who know us best and to seek God's leading. Ultimately the decisions are up to the individuals involved because they are the ones who will have to live out what they set up for themselves in the future.