For example, how did they find these participants? Did the researchers just pull married couples from the BYU campus or from across the city? Given BYU and Utah's population are they mostly LDS couples? What about their race given that the LDS is significantly more caucasian than the US overall? What about diet given that LDS members generally don't consume alcohol or caffeinated beverages? What about geography? Perhaps temperature or altitude affected the cardiovascular markers or measurements were taken under different conditions for different groups?
All of the above things I've mentioned may or may not affect cardiovascular health, and I doubt they captured or had the time and resources to control for all these factors and any additional ones I didn't bring up. So I think being skeptical is okay. Until this study has been reproduced multiple times or we understand the entire pathway connecting marriage and heart health (a highly unlikely occurrence) I will remain skeptical because that's how good science works.
And you need a representative sample to capture the variance of a population. The smaller the sample, the less likely you will be in capturing the actual variance.
I think if you want to generalize (which is very rarely done in science) then the sample size has to be representative of your (general) population.
Now we know that media (i.e. NYT) likes to take the extreme specificity that makes studies significant and apply it broadly. That's really where my contention comes from.