Both the Bible and the Catechism of the Catholic Church explicitly forbid sex outside of marriage. But cohabitation is not a guarantee that fornication is taking place. So to say that “Christians are not to live in such a manner,” without exception, implies that I should be able to find something to that effect in the Bible or in the CCC.
To my knowledge, there is nowhere in the Bible or the CCC where cohabitation (without fornication) is mutually exclusive with a Christian lifestyle unless one of these two things is true:
- Cohabitation would increase the temptation for sin (of any kind) versus non-cohabitative living arrangements.
- Cohabitation would cause scandal.
If you told me that one or both of those criteria were true for 99% of couples, making cohabitation sinful for those couples, I would not challenge your data. However, if a couple is not fornicating and prayerfully confident that their living arrangements do not satisfy either of those conditions, then I don’t see how we could condemn their actions. I’m talking about some serious confidence here, like the quadrapalegic who can’t have sex living with his caretaker. But without a solid understanding of how an individual is tempted and without knowing the effects that their cohabitation would have on their particular community, I don’t see how we can pass judgement on their actions.