Well, I don't think they are based on the incredulous looks I get when I talk about it from nearly everyone. Really depends on where you live, in TX I'm assuming it's very rare outside of anywhere but Austin.
But someone deciding it's time to cheat and telling people they are in an open relationship seems like a good way to immediately get caught. Ya know, since people talk and that's the kind of story everyone wants to share. "Guess who's in an open relationship" "Wait what? That's not what his wife says!"