I personally find the party scene very entertaining, and I assume it is currently written the way it is to let players know that "hey, all these characters are romanceable and this is the time to start it". I suppose how open the discussion is on who you bed and what everyone's opinion is on that is off-putting to people, but I speculate that there is an aspect of mindreading among the partymates that isn't as overt as writers intended. There is no privacy in the party because of the tadpoles, and so they make it their business. Additionally, it should affect other partymates if you favor one of them over the others - especially one who is on the opposite morality aligned to them. Tbf, I can't tell if some of the disembodied approvals/disapprovals are bugs or a feature.

I wouldn't mind the addition of more friendly dialogue options that can friendzone the relationship, but the option has to somehow be both natural and clearly marked so you don't lock yourself out unintentionally. Aside from two characters, offering an additional line of dialogue thanking a character for remaining in your party as opposed to saying "you don't like them like that" allows players to roleplay naturally as friends without attraction to each other. Unless of course the writers intend for certain characters to develop feelings for your character no matter what at certain approval levels. If they are written that way then they should feel spurned, though I assume they'll get over it.

As for Lae'zel and Astarion, because this moment in the game is the start or at least a taste of romances, and while it won't be as edgy or extreme as it could be, let's not kid ourselves about what "romance" means for these characters. These are characters who fall on part of the morality spectrum where trying to hug them is as bad as calling them worthless and being sympathetic to the downtrodden triggers them. You can only imagine what that means in the context of attaching to them. Their sexual hunger is psychological release and that's intimacy to them at this stage in the game. Romancing them most likely won't woobify them.
So, it makes sense for them to spring this stuff on you before you get the chance to properly turn them down. From what I read from others, its an important point in Astarion's narrative that he is the one who initiates, and is less interested when the player character does.

Aside from just not talking to either, one way to allow their scenes to stay true to character, but give players the chance to nip it in the bud is for their dialogue to be broken up. Perhaps this only applies to Lae'zel, but right at the start of her spiel (which gives enough to hint at what she's about to say), you could be given an option to "Let her keep talking" or nope out of there.