This may be true for married couples who started dating when they were teenagers. Since you brought it up, I'd be curious to see what proportion of the couples in the study fall into that category. I have a hunch that it's a pretty small percentage, though.

While it's true that there is some stigma against (e.g.) a college freshman or sophomore dating someone who's still in high school, I don't think many people in their right mind would object to a 2-, 3- or even 4-year age gap once both partners are at least 20.

EDIT to add: I don't think 17-year-olds being afraid to date 15-year-olds even registers on the top ten reasons "why people are increasingly lonely." Your take comes across as very high school-centric.