Originally Posted by Asri
This is a great point -- so I went and looked up the devnotes in the dialogue about the tryst with Mizora. For Astarion, they indicate he is being sincere saying he doesn't care about someone else getting a "nibble." Shadowheart is non-chalant and doesn't care. Unsurprisingly, Halsin is truly supportive of Tav's adventures.
Can't speak for Astarion. But Shadowheart does care for sure 1 way or another(how you look at it). Telling Tav to "not forget about her" is a bit icky while calling Mizora "inferior goods"(there is more to this of course, but there 2 things are the most obvious anyway).