Most people playing with her romance in mind will never think: "Hmm, Shadowheart must be poly" with everything she does and says outside of interactions with Halsin. Why? Because most people tend to play as monogamous. Besides, even if she were secretly poly it still isn't a good thing at all. I'd 100% like to know if she's into that kind of thing before so I don't get emotionally invested in her and feel stupid later.
She only mentions it if you encourage her to do so. I don't like the idea that variant SH pours the blood of the nightsong on the statue of Selune. But I never have to see that in my game even as I know she has that potential. Is potential for poly any worse than potential for true evil?
Let other have their fun in the way they choose to do so - my choices shouldn't ruin your fun.