One of the things I never, on principle, ask my students about is their religious background. Nonetheless, I sometimes learn it about at least some students - in the case of Religious Geography of New York, I learned it about most of the students - and I noticed that many came from what could be called "non-practicing mixed religious marriages." (Jewish father, Catholic mother, etc.) This explained, I thought, their interest in religion - what was it that their parents had left and why - but also their desire to find religions compatible. This might be right.
But I was wrong to think this distinguished my students from students everywhere else - at least on the mixed religious front. (For the record, I'm the child of parents of different faith traditions myself.) It seems that religiously mixed marriages are common in the US, and have since the 1990s even become a sort of norm. Thus, at least, Robert D. Putnam and David E. Campbell in American Grace: How Religion Divides and Unites Us (NY: Simon and Schuster, 2010), 134-60. A fascinating trend, with hard to spell out implications for all sorts of things...