Non-ethical could mean without ethics not in violation of ethics?
I guess that would mean the relationship has not or could not be defined by petty human rules of conduct. Like telling a supernova or a black hole to clean up it's mess.
It's a piss take of the ethical non-monogamy apps.
But if we're being a bit real - nonconsentual arranged marriages, 50 year age gaps, marriage for money, marriage for convenience etc etc would fit the bill
"situationships" I think is the key operator. Where there's probably a power dynamic, one person is being led on/getting played, and the other person is the player.