Absolutely agree, and nowhere do we see this more often than issues of sharing.
Experience has taught me, when I see a thread on the theme of "Master wants to take another slave," to expect a tragedy. All the more painful if the sub honestly tries to live up to the impossible ideal of "I can be happy with anything if it pleases Master." I have seen some working themselves into a breakdown that way.
Not that all slaves are obsessively mono, far from it: I've known many who truly are glad to welcome anyone else who makes their loved Owner happy. (My ex-slave made only one absolute condition, that nobody should be put under her orders: she didn't mind how many other slaves I had so long as she was the lowest.) But poly and BDSM are two separate orientations, and one doesn't imply the other.