This may just be a "hot take" and this is solely my opinion but, I disagree with at least the last few comments on this thread (though there's no real problem with them at all, we all have our own preferences):
In my opinion, a lot of the immersion I get from a Role-playing game that includes romances is when they make the characters as close to a real person as they can. In real life, everyone has a sexuality, everyone has a preference and when a game gives certain characters certain preferences, I start to see them as believable characters. The "player-sexuality" aspect of this game kind of breaks that immersion for me in the sense that no one seems to have a gender/sexuality preference and every player character is appealing regardless of who they are. It makes it seem like the characters are just there for player consumption which, to be fair, this is still a video game so it's not far-fetched to say the whole game is technically that way. But I enjoy when good writing lends itself to believable character-developement. Certain characters in this game definitely feel like they should have certain sexuality preferences and the fact that they don't breaks the believable nature of them when it comes to romances. In addition, polyamory is a big step and big decision in most relationships and for a lot of people, it can be trying and emotional to tackle. Making everyone polyamorous with everyone just makes the character's sexuality even more unrealistic.
Now, this is just my personal opinion. If any of the suggestions for this were to be implemented, as long as there was a way for other people who don't want it to avoid that route, I have 0 issue with people having more options, more polyamory, more romances.
But when you get a line from Astarion when he's explaining a very personal aspect of his character and it includes taking sex slowly and that he'd prefer "no one think of him in a sexual way" for now, I have a hard time believing that character should be able to be polyamorous with anyone. It just feels like it goes against his character.
To each their own though! This is just my opinion and we all have our preferences
