I thought Astarion was hitting on me... Turns out he wanted to suck me off, just not in the way I was hoping.
My pansexual ass actually only had the scenes with Lae'zel and Wyll. Those were the only two that just happened on their own. The rest I've had to work at, and nobody likes me! 😭
Not all the companions are willing to accept a poly relationship, and if you lock in on one who isn't (like Wyll) everyone else backs off.
(Imagine my surprise when things got serious with Astarion in act 2 and Gale, who I'd never flirted with - I never even got that one scene that starts with the Mystra image - suddenly demanded I choose between them. Awkward.)
And Astarion is just hard to please in general if your character doesn't find chaos and a little death on the side fun.
I am currently wondering which companion(s) I can take into a particular optional encounter toward the beginning of act 3. Even the ones I've romanced don't have an option to ask. Unlikely to make any other friends on this dude given I am the Dark Urge and playing full evil. But... The Dark Urge should be able to satisfy this dark urge too.
Once you get the second romantic scene where you cana sk companions for a kiss, you are blocked for any other origin companion, although some don't mind sharing you with non origin ones like halsin.