Yeah I explained a common set of 'mechanics' I've read used other stories, but the general idea is just people fulfilling their sexual desires whenever and whereever with whoever they want openly. The rules on acceptance/refusal aren't rigid. The specific mechanics would probably stem from why free use has become a thing in the world, why sex isn't all that taboo or inherently private in this universe and seen as a common, everyday activity akin to getting a cup of coffee.
For example there may have been some kind of of catastrophic world event that has caused free use laws to be passed in order to help repopulate the human race. Or maybe something completely different.
It's more of the start of an idea, and how it gets applied in a story is up to the writer.
I actually started this thread to see if there were any suggestions people had that I could use to write a free use story in an established universe. Like I was thinking "what if I was watching the show Friends but it was set in some kind of free use society". But then I didn't really want to write an episode of Friends...