It seems like you have made a case that marriage or divorce need only consist of an internal attitude of heart among the parties in a coupling, so that any committed couple would become divorced in God’s sight at the point where they no longer wanted to be married, even if it were an unspoken feeling. Do I have this right? If so, then since this state of affairs can develop involuntarily over time in spite of the best intentions, how can such an ‘internal’ divorce ever be considered a sin? There is no act of violation, but only a falling out of love.
The flaw in your thinking comes from the fact that love is not just a feeling. It’s also an action. When a couple commits to love one another until death parts them, they’re not saying they’ll always have that initial feeling, they’re saying they’ll always take the necessary action. The necessary action is described in Ephesians 5:22-28, Colossians 3:18-19, and 1 Peter 3:1-7. As a blessing for taking the necessary action, God will help them keep the feeling alive.
Many couples who don’t do this wind up falling out of love. They often maintain the outward appearance of marriage for various reasons, but in their heart they may secretly harbor a raft of feelings such as resentment, anger, envy, lust for others, etc, in place of the love they once felt for their spouse. These are all sins, and if not confessed will give the devil a foothold in their minds (Ephes. 4:26-27) which he will quickly turn into a stronghold. The result is often an apathetic husband and a rebellious wife. In their hearts they’re divorced.
A situation like this doesn’t happen in spite of the couple’s best intentions, because that would mean they did everything possible and God didn’t keep His end of the bargain. It happens due to a failure to take the necessary action. He will not dishonor those who honor Him.
God has never been willing to accept external appearances in the absence of internal motives where it comes to worshiping Him (Isaiah 29:13) and since marriage is supposed to be a model of the relationship between Him and the Church, I believe the same principle applies to our marriages.