Someone who was some years older than me and had been a hippy once told me something like "TV went off at 9pm. There wasn't much to do after 9pm other than sex and drugs."
We now have the internet available 24/7. You can have live conversation with people around the world at any time of the day or night. You can get a lot of your sexual and emotional needs met through some means other than a relationship in meat space.
Historically, the default assumption was you got married and presumably the result would be children. We had a lot of cultural norms, like shot gun weddings, to deal with the reality that birth control mostly wasn't a thing and there were consequences to getting your needs met. This was part of why we generally insisted on standards like no sex before marriage.
A lot has changed over the years. For good and for bad.