In short, the enlightenment privatized marriage, taking it out of the public sphere, and redefined its purpose as individual gratification, not any 'broader good' such as reflecting God's nature, producing character, or raising children. Slowly but surely, this newer understanding of the meaning of marriage has displaced the older ones in Western culture.
Most apologetic books are really written for Christians, even the ones that purport to be written for non-believers.
One of the most frequent responses I get from non-Christian readers is: 'I'm not sure I agree with all this, but I must say this is the first book I've read by a Christian that didn't treat me like I was an idiot.'
I am a Christian resident of New York City. I simply read things the other Manhattanites read (NY Times, New Yorker magazine, Wall Street Journal, and many of the books they read) plus all my Christian reading. I don't do anything special to understand skeptics. I also talk to a lot of skeptics and read things they point to.
I wouldn't venture to say which kind of sin is more prevalent. I wouldn't even want to try to characterize certain 'circles.'
Big cities have a lot of 'younger brothers' who have left traditional parts of the world and their families for a more liberal lifestyle. But cities are filled to the gills with 'elder brothers' too.