...As Kate Middleton and Prince William have thrilled the British public this week with the news of their engagement, there's been barely a whisper about the fact that they've been co-habitating on and off for years.

"It's amazing how little one has heard about the co-habitation issue," said Deborah Cohen, a professor at Northwestern University who specializes in modern British history. "The modernization of the monarchy has developed very quickly in the last 30-plus years on those sorts of social issues."

Historically, societies began to prize virginity as a marital attribute when they started to develop inherited wealth and social status, said Stephanie Coontz, author of "Marriage, a History: From Obedience to Intimacy or How Love Conquered Marriage."...