Without commitment, there is nothing to have fidelity to. Chastity, meaning no intercourse outside marriage, was the unspoken standard among Christians until very recently. The idea of “going steady” is a recent invention. Of course, the open ended multi-year “engagement” is now common.
I think a lot of people have this idea that until the 50’s 60’s (or whenever) people were not promiscuous and having sex.
Based on what I have seen in my life and what my folks and other older acquaintances have told me things have not changed that much, it’s just in more recent times it’s been more openly accepted and or discussed.
In reality it’s a lot harder to hide these days with our pervasive technology.