At what point do you believe that a marriage, while still legally valid, is no longer meeting Gods idea of marriage?
I'm usually the blacksheep around here when it comes to this. I do not believe in "once and you're done" - mostly because extensive biblical study and challenging my own beliefs in this area has shown me otherwise.
You already acknowledge that a marriage is still legally valid, while it may no longer meet God's idea of marriage. I think to answer this honestly, you have to figure out what God's idea of marriage is. It's late and would require a lot of looking up scripture, but in short, it is the promise of mutual love, mutual respect, mutual submission, mutual service, mutual help, and sharing in carrying each other's burdens. It's about doing life together, and committing to doing all these things. We need to treat our spouse the same as - or preferably better than - we would treat any other brother or sister in Christ.
When it is no longer a marriage is when one or both withdraw this commitment. Commitment is the combination of care and consistency. If you withdraw either, you are left without commitment - that is, the promise that is part of the covenant. We are shown over and over in the Bible that marriage is a bilateral covenant. God made a unilateral covenant to Israel and it turned out badly for him. For Jesus, his covenant is new...it is bilateral, as only those who accept him will be his "bride." Marriage requires two who are equally consistent and caring with each other. Lose that, and the marriage is still "legal" but isn't much of a marriage.
Sex can still be had, though, even if there isn't much of a marriage when still legally married. Sex is a drive, so while it is the seal of the covenant, it is not the marriage itself....though, at my age, I wonder why anyone would want to be with someone who they don't care about, or who they know doesn't care about them. When you're young it's drive... when you're a little older, it is far less important.
In short, I believe a marriage is no longer a marriage in God's eyes, when the couple decides it's time to divorce. But imo, the marriage is not over until the divorce is final. As God gave Israel a certificate of divorce to finalize his divorce to her, so I believe it is necessary to end a marriage in his eyes for people.