She (and also other demons) are supposedly capable of transforming at will. They mostly don't because the form you see is most likely the form they most desire to have, but Kas, at least, is extremely skilled with transformations, so she can just keep it however she feels like.Tight? Kasyrra? Not to slutshame the succucow, but I seriously doubt her vagina is tight.
In the palace itself, when you hug Kas, one of the companion reactions is:Also, doesn't Brienne get pissed after the winter palace Kas romance path?
But how she reacts afterwards, when you talk to her in, say, the tavern, it's this:"Really?" Brienne huffs. "I'm not cow enough for you anymore?"
Basically, no? She's not pissed. She's concerned for your health, but she doesn't mind the pc fucking Kas.