They were together when she was 16 but married when she was 19.
That really doesn't make it meaningfully better!
I agree your sister was likely looking for an escape from a dysfunctional home life
Op I think your dh is right you are naive
You are right that it's creepy and gross!
Men are mostly creepy and gross ime
From the age of 13, csa at home and many, too many, much older men "trying it on" with me with no sense of how vile they were being. Really made me feel like it was something about me that "invited" this crap! Reinforced by being told by abusive father that I was a "flirt" and "tease" and blamed me for the abuse - still does!
The law makes SOME Of them check themselves but most are far from aware of just what disgusting creeps they are
They've been together happily since then.
Yea cos coerced and controlled girls/women are always aware they are - not! 
You have an odd idea of this set up I'm not entirely convinced you're describing your sister myself
My dd is now dealing with the mental gymnastics of dealing with the fact my ex's 2nd wife who was ow was 2 years younger than she is now (almost 20) when they got together. By the time ow/2nd wife was dds age she was pregnant with their 2nd. By all accounts it's very much an "uneven" relationship in that she is very controlled by him. He wouldn't have got away with that shit with me! I'm less then 5 years younger than him and pretty assertive perhaps due to previous experiences. Dd currently "reconnecting" with her father but warily so. Gets along great with "stepmum" but then they're fairly close in age so hardly surprising
The most popular "categories" in porn viewing tend to the very young and incestuous too.
I think it's mainly a case of their being self protective enough not to mention what they really think/feel to anyone who might carefully monitor them!
Woody Allen is notorious for this hardly a shock given his own "predilections" don't you think?
I refused to watch and therefore support anything he has been involved in, ditto Roman Polanski Who should be rotting in hell frankly
It's probably different for those who work with teenagers though or those who have daughters of that age
Why do you think that? Ime it makes no difference
Men are ime still perhaps even regressively so predatory. There are evolutionary reasons for that - doesn't make it right.
Lot of naive and defensive posts on this thread