I seriously wonder how you propose a study like this for federal funding with a straight face? Gallup is probably a sex obsessed scientist who comes up with 20 such proposals a year. The guys at the funding review board are probably like "Uh-oh, another one from Gallup. Hey guys, look at THIS one...."
Janine, they did say that they looked into other possible explanations, including strength of the relationships, and personality types, and neither of them offered a statistically significant explanation. One might wonder, actually, if abstaining from using condoms is WHY these women might have longer relationships, rather than the other way around. At least from my personal standpoint, the study just shows that the natural way is probably best, as usual. If it does have this effect, it shows that mother nature has more built in ways to ensure the survival of the species.