Personally not a fan of the whole orgy/threesome thing at all, but I get your point. For Astarion, I can almost say without a doubt that it's in his character that he only takes part because he's afraid of losing you, therefore making it easier to view his orgy disapproval as a bug. With Shadowheart, it's way different. It's been brought up time and again, but the reason why Shadowheart does a 180 on Halsin in Act 3 is 99.99999% because they have the same writer. It's been brought up that Halsin feels like somebody's self-insert, which isn't difficult to see how people can come to that conclusion.
Astarion's only up for it
after you deal with his quest, so at least he really wants to try, with or without Halsin, now that he's finding what he actually likes. So you can be like, ok let's do this, but of course the game decides to make you feel like an asshole when it tells you he dissociates and you can't do anything about it.
Act 3 is just us living out somebody else's kinks with Halsin, Mizora, some tentacles, Haarlep, and Ascended Astarion (that one makes sense at least).
