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.

My wife is 9.5 years older than me. We met 10 years ago when I was 24 and she was 34, moved in together 6 years ago when I was 28, and got married a little over 2 years ago. I've never dated anyone my age or younger, I've always tended to date (at least for longer than a night to a few weeks) women that were at least a few years older. I have never felt any sort of stigma against it, quite the opposite actually. 2 of my best friends ended up with women 10 years younger than them, and for that matter very few people I know are with someone within a year of them, typically there's a discrepancy of about 5-6 years, and these are all people ranging from late 20s to 50s. Heck, one of my really good buddies is 55 and has been with his 72yo wife for over 20 years. I'm guessing this is a younger generation thing, cause it seems odd to me to that age really matters once you're past your early 20s.