My concern with married crew members is not with the potential for conflict. Quite the opposite -- I fear the feeling that "we know each other so well that we don't have to *say* everything." This could be a significant barrier to operational communications.
As many of you know, my flying experience was in a corporate aviation environment. There, it is common to fly with the same person every day, often for months or even years. It is close to being married (with some obvious exceptions), and over time you DID get to know how the other person thinks and reacts in certain situations.
I can think of several cases where we made assumptions about one another's thoughts and actions which had to potential for causing trouble. Gratefully, they never did.