How deranged are people to assume that Astarion is mean to his lover after he ascends? This makes me think they never romanced AA and never tried to not cause friction with him. AA is VERY romantic if you let him be. Is it toxic that you can't argue with AA? Hell yes AA can be toxic if you argue with him, but the option is still there to keep him happy and acting like he utterly adores you. The people who have never experienced this still feel like they know what we're arguing for.
You can determine AA's behavior as his partner. Is it wrong that you have to behave yourself for him to be loving? Sure, perhaps it is, but it doesn't change the fact that he IS loving if you allow him to be. Whether it's an act or not is irrelevant. It's how he has been portrayed up until this complete character change with the AA kisses.
Having him be toxic when you argue or disagree with him I quite like, it makes his character deeper and more interesting. I absolutely agree with you that AA is very romantic and he does care very much for his new consort, most people who do not romance him never see this side and probably would not get why anyone would want to romance him. Also it is a roleplaying game, something wrong in real life might be fine in game because it is fiction not reality. This does have it's limits though, when hurting the players with (sudden in-your-face) abuse for instance.
And to make it clear again, it is abuse bacause because of Tavs reaction and facial expression, not necessarily Astarions animation.
