I know a unmarried couple who have been living together for 30+ years. True definition of soul mates. 3 children. When you hear the stories about a spouse dying and then the other spouse dies the next day due to grief, well this will be this couple.
Then you have 50% of marriages ending in divorce and probably another 25% of marriages that are unhappy but don't get divorced.
It's not as clear cut as you want to make it out to be.
It is clear cut to The LORD, and He is the one with whom we have to do. These people need to get themselves right with Him and also their relationship with each other. If they love each other and are committed, especially with children, why haven't they married?