A general question is to what extent parents should try to influence and regulate the personal lives of their grown-up children.
This thread reminds me of an article
http://bucks.blogs.nytimes.com/2011/07/06/living-together-pays-off-more-for-the-college-educated/July 6, 2011, 3:21 PM
Living Together Pays Off More for the College-Educated
By ANN CARRNS .
It may be that high-IQ people (college grads have higher IQs than average) are more able than others to make good decisions in their personal lives without much outside advice (including that offered by organized religion). For most of U.S. history, and currently in much of the world, cohabiting outside of marriage is "breaking the rules". When rules are relaxed and people are left to their inclinations, that may be good for the bright and bad for the less-bright.
In our household my wife will likely lay down the rules for our kids, since she is more conservative. That leaves me free to philosophize, but not in front of the children
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