Cait is a good 'insert slutty slut here' character, in a way that none of the other companions really work outside of high libido Ryn. Arona only bottoms for you (and Domrona not even then), Atugia's too reserved, Azzy seems to be only into people she has a romantic attraction to (champ and Lia), Berwyn's in flux as a character and thinks he's a top, Brienne is only into you, Brint's mainly a top, Kiyoko is only into you, and Quin again is mainly a top.
Add to that Cait is a character that most people playing will have available, and you can see how she's the one who gets slotted into threesomes, orgies, defeat sex, etc.