Cohabiting couples tended to be less educated, younger, had a lower household income than married parents, and the quality and stability of their relationship also differed, it added.
If fullness of life could be found in sexual stimulation, or if it was just a matter of making babies, the “forget formality and just have sex” approach might temporarily satisfy cravings and cause enough conception.
The findings are likely to be welcomed by Children's Secretary Ed Balls, who has led Labour's campaign against the Tory proposals for a marriage tax break.
Mr Balls said in January: 'Once you adjust for the fact that people who are married tend to marry older, be better educated and have higher incomes, you find it is not the legal form, it is the strength and stability of the relationship which is most important.' The Government's Office for National Statistics said last month that married couples are more stable than those in other types of relationship no matter how well off, well-educated or old they are.
The happiness of marriage is not only or even mainly physical.
With the sex, there ought to be a deep sense of safety, a sense of being loved and accepted for who you are, a desire to please without the need to impress.