How do you know if a guy just sees you as a potential "side piece"?

How do you know if a guy just sees you as a potential side piece?

Hi Guys and Gals,

Ok, here goes. A friend has been claiming to be "more than a side piece" to the guy she's been seeing... and yet... she's apparently one of a few contenders for the hot seat of "Future Mrs. Dbag" to a fellow who is still married...

I've been trying to explain to my friend that she's better than this... worth so much more... and yet she's just letting herself be in this situation, convinced that no matter what else she's seeing SHE is not "the side piece."

The sad thing? She's super highly educated, has traveled basically everywhere, is extremely charismatic... has EVERY F-ING THING GOING FOR HER. Why not just go out there and pick an available guy without all these complications and attachments?

I brought this up privately to a guy friend who then told me, "It doesn't matter if the guy's married or not. She's only side piece material."

?

So I'd like to ask y'all a few things:

(1) GUYS & GALS: WTH qualifies as "side piece" material?

(2) GUYS: How do you treat a woman you see as a side piece?

(3) GALS: What makes you think a guy is treating you like a side piece?

Updates
10 mo
*To clarify* --> Apparently some girls "just seem like side piece material" to guys (even to single guys). I'm trying to figure out

(1) What makes a gal seem like she's up for that sort of thing.
(2) How a guy's behavior indicates he sees a gal as a side piece.
(3) What makes a gal think that a guy is possibly treated her as a side piece.

(Ok, I've officially exhausted my use of this term for the rest of the month. 🤮)
How do you know if a guy just sees you as a potential "side piece"?
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