I agree with what you've said so far, but I also think there is a valid reason to be called a slut by my owner, as good_girl pointed out. If I think about sex often and do whatever my Mistress wants, it's true that I'm a little slutty... so when she calls me HER slut, it's okay. I do not think anyone should be called a slut by someone other than his/her owner. But for some, the fact that it is seen as a humiliating or degrading term is what they like. And if the shoe fits...