The behavior of drinking coffee or not is also very socially mediated-- portrayal of coffee drinkers in movies, coffee as a common activity for preliminary dates, or to meet someone new at a cafe...

All of this means that coffee drinkers are going to self select for various other features that add confounding factors. Imagine you were doing a study on marriage patterns: I wouldn't be surprised if "Drinks their coffee in the cafe itself" was more highly associated with being single. And the headline will read "Dedicated coffee drinkers less likely to find a partner"