Shadowheart might not be willing to share at that point because/she/ doesn't take you to bed herself yet.
This would be hypothetical at best because it's never implied to be the case plus she clearly states she wants you alone regardless. But then you have to remember that the Halsin proposal happens before her final romance scene and suddenly this argument is void.
That said, some minor rewriting and making Karlach an option for open relationship would be the obvious first step to fixing this inconsistent stuff in a direction where she would work as poly, though I'd argue sharing someone you haven't bedded yet is.. odd nonetheless. And then her devotion in act 3 comes out of left field instead.
The problem isn't that Shadowheart is poly, it's that she's so very inconsistent about it. I at least want her character to flow better...it's clear Shadowheart has got some serious kinks to satisfy but only rarely reveals them.
Other than the Abdirak scene, when is this actually implied though? The drows? There she's out of character in the first place. And during the Abdirak scene she still has fond memories of her actions at the House of Grief, the torture she inflicted and all that, due to having been recently mind wiped into thinking so. Which is confirmed upon first meeting Raphael, and upon failing the first persuasion check in her first romance scene and some other scenarios I can't recall exactly.
At the House of Grief she starts remembering and noticing the less pleasant parts of it and is disgusted by her actions. Then Shar brings all the negative memories back. After that I can't fathom she'll look at any of those scenarios with anything more than PTSD. It'll just remind her of what she did to her parents.
Either way, that's theory. Reality is at least that she calls the epilogue night together 'perfect' and while we don't get to see what happens there, it's clear we're capable of satisfying whatever she wants.