That's dissapointing. I wouldn't expect them to be able to work in a ton of dialogue for every character pairing though. I was thinking more along the lines of being able to have a poly romance with two specific characters who would be up for it. Assuming they're initially designed to be romanced seperately anyway and as long as most of their scenes don't conflict, they wouldn't have to work in that much dialogue. Just add in a sweet, end-game, alien threesome. They did it with Jade Empire.
The first problem with designing a designated couple to be open for polyamory with the PC is choosing the gender(s) of said couple. I might expect they would choose a male and female, but can already hear the complaints that it wasn't 2 females or 2 males (and in ME, you can add alien preferences to the mix).
There are sometimes places in the game where the PC's relationship is acknowledged by other characters, or the LI has a specific role in a cutscene. It could be very costly to do that if the LI was plural - not only in terms of needing to design multiple versions of the scenes, but it could also limit the kinds of scenes they could write.
There's usually a romance lock-in conversation in every game, and those would need to happen with multiple partners together.
I remember one particular DAO playthrough, where Wynne was having a chat with the Warden about her relationship. She was actively romancing Leliana, but had enough approval points with Alistair for him to have given her the rose. Wynne's lines were schizo - she'd speak of Leliana in one sentence and Alistair the next.
There was a series of cutscenes late in DA2, where Hawke could go around and speak with each follower prior to the final battle. The conversation with the LI included a kiss and some romantic dialogue. In this case, I suppose a poly pairing would include a 3-way romantic conversation?
I have a bit of trouble imagining how they could reasonably do polyamory without somehow misrepresenting the power dynamics. If, for example, the player frequently took partner A on missions and got all of partner A's romance dialogue, it could start to feel like the real relationship was with partner A, and partner B was just there as a sex toy.
In any case, I think the investment required to do it right would be far greater than any actual value it might add to the game.