That's an interesting observation and I wonder if that idea comes from the fact that, if this extended friend group is generally comprised of people in their mid-30's (Joanne and Larry excepted, I guess), they aren't exactly the youth culture anymore. These are people that would have been born in the mid-1930's and grown up in the 40's and 50's, right? Maybe they are retaining their 1950's values, even as the culture is being changed by the generation below them.
Of course, that said, this is a show that does have a couple of unconventional relationships. Peter and Susan divorce, but stay together. And Joanne and Larry seem to have some kind of open marriage. And there's a scene when characters smoke marijuana, which was likely not a big deal for people in their 20's at the time, but, again, these are not those people.