I don’t know. If you have a generation that says it is wrong, but only give social reasons, and doesn’t back it up as a moral wrong, then I think the generation of people that they raise will use this information to analyze their situation socially only, and not morally. This will lead to most of them choosing to live together, because they only know it as wrong for social reasons, and not moral. So if it seems to cause no harm socially, they move in. The only thing they have stopping them is that their parents don’t like it. It’s not like they have learned it is wrong, as in WRONG. They weren’t taught that living together is anything on par with killing, for example.
Of course, once enough couples are living together, it becomes difficult to see the social wrong on an individual level. You still might, though, with statistics.
The question is, why did the original generation only explain it in social terms, and not moral? Had learning it on those grounds always been sufficient before?