I agree that part 1 is bunk, but I think there's one factor they're neglecting to mention or ignorantly leaving out: this involves couples that cohabit IN LIEU of marriage.
I've lived with a girlfriend under the guise that we would have separate rooms and just see how we live together. The first few months involved sleeping in the same bed and lots of sex. The next few months involved a lot of weirdness. The last few months were just like we were friends and nothing more. It drained out of us, the novelty of living together, and we kinda realized that it was better to part ways.
If you live together as an alternative to marriage, I can definitely see how that would cause eventual "divorce." If it's innocent cohabiting to have another person helping to pay the bills, and there are no kids in the equation, I don't see a problem. If anything, it seems counterintuitive to say that cohabitation leads to divorce, but who am I to say their study is bunk. Hell, they keep saying a majority of Floridians want Rudy and McCain to run for pres... they don't ask me, cuz I'd never say that!
"The next few months involved a lot of weirdness."
Precisely.
Jiminy Cricket tends to suddenly appear after the "passion" dies down a bit.
We Christians tend to call it the Holy Spirit's "voice of opposition to sin."
Whatever you wish to call it, it does appear, and then things will get CONFLICTING and UGLY.