The one thing I thought Thaler and Sunstein were most wrong about in Nudge was the idea of privatizing marriage. They propose that the state "get out of the marriage business," offering only legal civil unions. Marriage would be left entirely to religious institutions.
Their account of marriage misses the fact that marriage as a social institution is not primarily about the feelings of the married couple, but about the best arrangement for raising children. Marriage works best for kids, and produces many of its benefits for married people, because it is a permanent, socially recognized and supported institution.
Marriage is not a nudge, but a permanent choice to change yourself into a part of something larger than yourself.