> Polygamy isn’t technically legal or common, but you still have plenty of people who have many romantic partners—just not at the same time.

I don't think those two are the same. Polygamy is a way to structure society and determine access to resources, whereas fooling around on Tinder is just a way to have fun with no questions asked. It might surprise you but the plebes of old were also promiscuous.

Here's an interesting discussion on the topic:

https://www.reddit.com/r/history/comments/5rre85/sexuality_d...