I think loyalty raises a lot by making them orgasm... like this I can get even captured rebellious slaves in my group after just one interaction. The lewdness rises a lot too, but cant keep up with loyalty.
maybe just unlocking the sexual action and sending them to brothel may get it (not sure if they get lewdness there, but they should).
btw, you can get both traits (Devoted and Slutty), its just hard to get the slutty one. You can use the regression potion to get the piliable trait again.
I once tried to get every stat up to 125, but stopped cause I couldnt get the slutty trait no matter what I tried... and cause going outside with slave courage base stat above 100 will reset it back to 100 (not sure if its a bug), so I got bored and stopped playing for a while >.>