I think it's likely more of an open thing these days, not necessarily more of a thing. The term itself was only coined in the '80s, and prior to that, I'd imagine talking about it was even harder (but it definitely happened; William Moulton Marston, creator of both Wonder Woman and the lie detector, lived with his wife and their mutual girlfriend). I also think that, while I'm not inclined to ever call anyone's sexual life "just a phase," some people might indeed explore having multiple partners before settling into marriage.

And, of course, in the "old days," someone might indeed be openly dating multiple people, only choosing to "go steady" after a while, but that wasn't considered a form of non-monogamy.