The answers, from the abstract seem to be "young people" are living at home more, drinking less and gaming more than they used to. "Among young women, the decline in the frequency of drinking alcohol explains about one quarter of the drop in the propensity to have casual sex. Among young men, declines in drinking frequency, an increase in computer gaming, and the growing percentage who coreside with their parents all…
I'm quite surprised to see that the leading theories in many of these studies revolve around competing forms of entertainment. I'm personally more concerned with how economic difficulty, mental health, and hormonal imbalances could be causing these sort of changes in society.
So I think the focus is about what forces were not present in previous generations, and the Internet is certainly the most conspicuous factor. I'm not saying it's the decisive factor, but I'm not surprised it's a prime culprit.