Back in the pre mid 70s, the elephant in the room was so much bigger than it is today.   So many of us got interested in having sex with a certain someone, and if the word got out that we were doing so, parents and peers felt no hesitation in letting us know that sex really should mean marriage.
The pill became widely available in the mid 60s, and the media was full of discussion about how CASUAL SEX was happening EVERYWHERE.   However, in real life, my anecdotal recollection is that sex didn't get dissasociated with marriage until the mid 80s.
Consequently, lots of us got married because we liked sex.   The divorce rate soared when people first found out that there was an alternative to that.    My 1976 bride and I were lucky to ride through all of that.   Our 45th anniversary is next month!