Sexual liberation is a big part of why this changed. People taking advantage of dumb younguns.
I'd be suspicious of a 40 year old dating my 20 year old because why aren't they married yet? What is their intention? Is my son or daughter going to be a notch on their bedpost? Will they be traded for a younger model later?
Or even more worrisome has my child been taken in by modern hookup culture behind my back and is the middle aged one going to be a notch?
I was zogged out when I was young and I kept it hidden from my family. Regret it now, regret taking advantage of 30 and 40 year old divorcees, and would regret not stopping them from making the same mistakes.
I wouldn't stop it, but 18-22 would probably get a glare from me. A camping trip and a gun in their face to make it clear what will happen if they fuck and forget.