Well, I know I tend to want to look at things just a bit off the beaten path and from way outside the box, so I will give this one a try!
I know for a fact that it has every bit to do with the lack of a right of passage into adulthood in our culture! The fact that Mom & Dad are too bussy making a living to provide proper guidance with many families also leads to what you have termed "Marriage Partners", as they usually get married to get out of the house. This true lack of Maturity has nothing to do with age, but with a lack of being taught a value system as can be found within most SPIRITUAL gathering places. Teens become Lovers by having sex, and not realizing that a marriage is intended to be a bond of friendship not just lust! Do you realize that Divorce has become the number one right of passage into Adulthood in the American Culture? Many think it is the drivers license or turning 21...
I guess the easy way is to say that marriage partners have sex, while SPIRITUAL PARTNERS have SEX. What passes for an intimate relationship with marriage partners isn't even good foreplay when compaired to that of SPIRITUAL Partners!