I've heard people claim that cohabitation is superior to marriage because life with a girlfriend or boyfriend is more romantic and exciting than life with a spouse. If this is true, then I would expect cohabiting couples to have sex more frequently than married people.
The MIDUS Study asked people about their relationship status--cohabiting, married, etc.--and also asked about frequency of sex. Answers ranged from "two or more times a week" (6) to never (1). Age matters so I limited the sample to people in their 30s. The mean for 250 married people is 4.58; it's 4.30 for the 20 cohabitors. The difference is not statistically significant, but keep in mind that married people tend to have the disadvantage of more years together.