Originally Posted by Naturalite
"Polyamorous relationships are intimate, meaning that there is some depth to the bond and it's more than just a casual fling."
You are over-thinking it.
How would you know if someone who had *not* been in an open relationship felt something for you? Would you ask them directly? Would you infer it from how they behaved with you? Would you send a note asking them to check which box would be appropriate?
Don't get hung up on the idea that someone is or has been non-monogamous, they experience love and closeness pretty much the same as anyone else.