"Why do you always assumed that you would want to have any kind of relationship with the first female character that you see in the game?" Well reading the overview we got this : "Your choices could impact your relationship with your lifelong friend to get intimate, stay neutral, or push her away romantically but that's not to say there aren't plenty of others to choose from!"
So if someone comes an read the overview of the game they will think they have a say in the relationship with Lilli but it doesn't seems like the case here. If for example, I try to sell you a potato, show you a photo of a potato, write to you an overview describing the potato and when you got it, it is a rock, would you still eat it? or would you complain? the same can be applied here.
Besides the way I understand polyamory (I could be wrong), you have a relationship between 2 persons, they decide to look for someone more, someone they both trust and want in their relationship, making that relationship that started with 2 persons a 3 persons relationship where all of them love and trust all the partners, and if someone wants to add another one to the relationship but the other partners don't like him/her it won't happen, because to them is more important the relationship of trust and love they got than fucking with someone that one of them fancy. I don't think that polyamory is letting someone you are with in a relationship fuck whoever they want, like I have seen in some posts in this thread, but instead all the partners need to be ok with adding another to the relationship.
But reading the comments it seems like Lillie the sister/best friend of the MC falls for Liam after he lets her some huge tips and only then she agreed to go to a date with him, making it sound like a gold digger, I'm not saying she is or that the creator of the game intended her to be that way, but it's the impression that some people got after playing it (I didn't play it because I usually don't play V0.1 of games but the renders seems good so I'm following the thread). So in a polyamory relationship, in the way I see it, would you trust a girl or boy that seems to be a gold digger? Because I wouldn't trust him/her and without trust there is no relationship.
So making one of the female leads and love interest, if not the female lead, untrustworthy won't appeal to people looking for harem/solo/poly/other too much. But that's what I think.