"The idea of the nuclear family (mom, dad and 2.1 children) living in their own house or apartment is the norm didn't really come about until after WWII."
You're correct. The norm was more the extended family prior to that war. Grandma and grandpa lived upstairs, or downstairs, and maybe the aging maiden aunt, as well.
It seems people have an uninformed view of America in the past.
The way we live in America today is different from how 99% of humanity has lived throughout history. Extended families living in one building was the norm, for rich and poor alike. Your family was your support structure. Kids would be raised by the family as a whole.
In such an environment, I imagine kids would have been exposed to sex at a pretty young age.
The homestead of your own has ALWAYS been part of the american landscape. It is the cookie cuter neighborhood that is new.
The "house" extends back to the start and earlier of american property law.
The extendended family was an excellent idea which would be nice if we could figure a way to bring it back.