OP, if it's icky for a 20 year old to find a 16 year old attractive, because the 16 year old is a child, why is it totally fine for a 16 or 17 year old to find that same 16 year old attractive? After all, the 16 year old has the same personality, the same body, the same life experiences.
It's not harmless innocent fumbling just because they're close in age - especially not these days given how much pressure teenage boys put in teenage girls to have anal sex (not much of an improvement on the 80s, that, if you're looking for 'grim').
Shouldn't you be just as concerned about any couple you meet who got together when the woman was only 16, because that means the dp/dh is someone who managed to find a 16 year old child attractive?
If on the other hand it's not the 16 year old being fundamentally not grown up enough for sex that's the issue, but rather it's all about maturity and manipulation and potential power differentials, then the 20 year old young man from this couple wasn't fundamentally 'icky' for fancying a child but was just potentially (and not necessarily) taking advantage of a power differential based on them being older. But then surely you've read enough examples now in this thread to know that it's not inevitable for a relationship with that age gap to be predatory?
I think you need to make up your mind. If a 16 year old girl is a child then she shouldn't be having sex full stop and any boy or man who wants to do that is 'icky', whatever their age. We should increase the age of consent. But if you're not arguing that the age of consent should be increased, and you're saying that of course 16 year old girls can have sex, the issue is different life experiences only, and power differentials, then I don't think you can be black and white about it because life experiences and power differentials are not black and white.