I'm just going off what I see in game. When you romance her, it doesnt include other people in her main romance. If they wanted her to be polyamorous, it wouldn't have been something optional, but something they would have made absolutely clear.
The fact that there's no consequences, no cause and effect, to doing the optional porn stuff is very telling. They should have done an ending where that stuff has actual consequences on their relationship, if they wanted it to look like it matters. But alas, they did not.
To be fair, they have shown they don't actually care about polyamory anyway. If they did, they wouldn't basically equate it to an orgy. They would have actual, meaningful poly relationships but they don't.
