... there's research that indicates it's not the sex that makes us happy, but the cuddling. And that happiness peaks at about one cuddle session a week.  So based on the numbers in the article you shared, we're still in peak happiness territory in general even with the decline in frequency.  Also, the article said a big reason for the decline is that people are too tired, and you can still cuddle even if you're tired.  

Sex is great, but if I had to choose going without sex or going without cuddling for the rest of my life, I'd stick with the cuddling.  I can take some things into my own hand, but it's hard to cuddle with yourself!