Statistically, married people have much more sex than single people. You are comparing sex in a bad marriage with sex in a (successful?) dating life. Two completely different things.
Compare happily married sex lives to happily single sex lives, and see what you get.
I am intrigued by your post. I have often wondered about these statistics. In the studies I have read (my high-brow reading includes Men's Health, and Ask Men), the control groups are single men of a certain age are compared to married man of certain age (e.g., single men 35-40 v. married men 35-40). And in every study, the married men are getting more sex.
I would think that if one could survey married people who are thinking about getting divorced within the next 12 months, and compare that to married people who are not, the married people who are not thnking about divorce would be having more sex. Not much science there and this is supported by the posts noting that women do not want to have sex with a husband they are angry with or do not care about.
When you say "compare happily married sex lives to happily single sex lives" and see what you get, the question means nothing. However, based on just the little seen so far in this post, it is really the woman's happiness in the relationship that is the controlling variable regarding the frequency of sex?