akuperma is referring to something much more modern, or much more small town. I come from a big city where my public school classes were almost all Jewish. My teachers were almost all Jewish as well. My parents come from a Jewish neighborhood, and it was more Jewish then. My grandparents even more so, and if you go to my great-grandparents, forget about it. Finding a non-Jew in the neighborhoods of 3 of those generations would’ve had the same odds as winning the lottery. Everyone in my line is Jewish, as well as all of my ancestors neighbors.
Neighborhoods were much more balkanized in ethnicity than they are today.
Conversely, in small southern towns. There was and is some mixing, but it was still not the norm. Southern history shows that Jews would send their kids from all corners of the state to meet each other and get married. It was not an impossible chore as it is today to not mix.
Lastly, I claim akuperma’s post about how all non-orthodox Jews should be considered goys is wrong for one more reason. I never even really heard of this whole intermarriage deal in any really serious manner until I I moved to what’s supposed to be a state with a lot of Jews and I started meeting tons of people who claimed to be “half-Jewish”. I’ve never heard of that one growing up.
There wasn’t much chance of mixing as there is in the last few decades. If all of your neighbors and classmates are Jewish, you’ll probably marry Jewish. If you can’t find any Jews around because everyone scatters, the kids probably won’t marry that way.