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Undisclosed? "It's complicated" seems more appropriate in that case.

Isn't "it's complicated" just Facebook jargon for "unhappily polyamorous"?

I've also seen it used to mean "I'm still sleeping with this person but don't want anyone to think he's my boyfriend."

Fucknemeses would also seem to be an appropriate use-case.

"Acquaintances who occasionally happen to bump into each other. Naked. With our crotches."