Even in this day and age there are people who marry because they’ve come to feel ashamed of “shacking up”.
That’s true.
The authors under discussion are, in my opinion, simply trying to make people feel better about sinning. Judeo-Christian history does not include a period before marriage where the couple lived together; it included a legally-binding betrothal period where the couple did NOT live together.
Part of the problem we face today is that there are no binding contracts. Engagement isn’t legally binding, and neither is marriage. In the situation, it’s not surprising that many couples don’t think marriage would add anything.