I have always believed in OSAS. but, how do we mesh OSAS with what Paul says in Galatians 5:19-21? I have 3 adult children who all profess to be saved but all are committing fornication I guess. They all seem dedicated to their mate but there is no marriage for any of them. Paul seems pretty definite when he says here that none of them who are involved in sexual immorality will ever see the kingdom of God.
In our culture, it’s commonly believed that people who live together without being legally married are “living in sin.” But it has not been that way for most of human history, nor is it that way in many countries today. And even in the US, 15 states still have “common law” marriage provisions. While the Bible only authorizes sexual relations within a marriage, it doesn’t specify the conditions necessary to constitute a marriage. It only says that a married couple cannot divorce except under conditions of marital unfaithfulness.
Therefore it’s reasonable to assume that the term “legally married” is man made and not a Biblical requirement. If your adult children consider themselves to be married, are acting like married couples act (being exclusive to each other, etc.), and are holding themselves out to the world as such, then in the eyes of the Lord they are married, and are not being sexually immoral.