I agree with the article but only if the people involved are in a happy marriage.

I wonder what the results would be if the survey was applied to civil unions or even polygamist.

One could say that we are happier in relationships because we are a social species, because this suits our psychology, but it is actually because relationships, in their deepest sense, affirm life. I can think of more than a few reasons right now why life shouldn't be affirmed, but it seems inevitable that affirming life is the way to go.