Quote
Because unlike in all other questionable situations during Astarion's romance, the game doesn't spell it out for you that this was not a good thing to do. And some people just find it hot.

In all the other scenes when the player initiates a situation with Astarion it ends up going badly for him (he disassociates with the drow twins, and dumps you after Moonrise Towers if you persuade him to drink from Araj/have sex with you). When he initiates something (sex in his final romance scenes) it goes well.

The polymance is initiated by the player (and Halsin) and the conversation with Astarion is uncomfortable to get through. It wouldn't be such a controversial issue if it was an okay choice in the game.