Marriage
Somehow I missed this (or else I've just forgotten I knew about it): there was a Gallup poll recently which "found [that] 60 percent of Americans no longer think sex outside marriage is wrong."
Also from the same article where I saw that: "In 1998, one-third of babies were born to unmarried women." (I'm assuming that's American babies.)
And: "The number of unmarried couples living together grew by 70 percent in the past 10 years."
For related data (with a strong pro-marriage slant), see the National Marriage Project: "Between 1960 and 1998 . . . the number of unmarried couples in America increased by close to 1000 percent." I think (from context) that they're talking about unmarried couples who live together (or, as they put it, "cohabit").
(And yes, I found those pages while doing research for a story. And no, it didn't answer my questions. Anyone know anything about attitudes toward marital sex in the 1930s? If I had more time, I'd try to get some info from the Kinsey Institute.)