we're talking about Babies and Weddings!!!! A lot of data relating to sex gets historically murky but babies are great because they are not only countable but we sort of have to count them. here is where the US counts them weddings similarly.
But I'm going to skip a lot of that (beyond noting how great the CDC data is to dig into) and go straight for the jugular.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1802108/pdf/phr122000073.pdf
[What you're looking for data-wise is the natural survey on family growth which only got started in the 1970s (and only within the last 20-25 years started asking a lot of questions more directly tied to this Q. The six page article above uses later data and the 1970s responses re premarital sex to estimate pre-maritial sex by age. Even if you don't fully accept that the trendline question is clear: young adults have gotten more promiscuous over the second half of the 20th century. This gets more complicated once we break the 20 year limit but that's out of scope. In addition starting in 1988 the GSS stated studying sexual partners
There was actually a fairly well reported on study from 2015 by Jean Twenge that attempted to answer this exact question using those two sources finding
“The median number of partners was one for those born in the 1910–20s, two for the 1930s cohort, three for (those born in the) 1940s, four for (those born in) the 1950s and ’60s, and three for those born in the 1970s, ’80s....”
essentially any way you cut the available data it is clear people born in the 30s were less promiscuous than Generation X and the Baby Boomers (especially since pre sexual revolution linkage between sex and babies was stronger. for a null impact you wouldn't expect that 1930 saw 1 in 6 1st births to women under 30 being ore-martitally born or conceived, versus 1990's 1 in 2).
this is a bit incomplete, there's a bunch of sociological research i've dug into about modern family structure but thats mostly in the 20 year zone so hopefully someone else can flesh this out
A complicating factor i'm not addressing is breaking down the numbers for subgroups that seem to be often implicitly referenced in these discussions. for example 1940s America is more than say Jimmy Stewart in Its a Wonderful Life. For instance: racial disparities are longstanding e.g. in 1930 (see babies link at top of comment) 94% of White 1st births were to married couples (or couples that quickly got married) while this was true of only 68% of Black 1st births.
/u/caeciliusinhorto has a British perspective/source in this old post https://www.reddit.com/r/AskHistorians/comments/32g3l6/what_did_people_know_about_sex_in_the_united/