I very much disagree with the idea that two people who don't want to be married should stay together just for the kids. Kids aren't stupid, they can tell if you're miserable and you sure aren't going to fake happy for a max of 18 years. Even if the parents don't dislike each other they are the child's model for a healthy relationship. If you want your child to have a healthy relationship in the future then maybe it's a bad idea to show them a loveless one for most of their formative years.
It's hard to quantify, but what if children whose parents are in a loveless (non-abusive, non adulterous) marriage have better life-outcomes than kids whose parents divorced because they 'just weren't feeling it anymore?' Law has to be written in ways that are supported by evidence, if we are to move forward.