I agree for the most part with this...in my experience even when I was in a typical D/s+
(one dom and two or more submissives situation)
...in all the ones that worked for any length of time: we girls often messed around with each other both in "his" presence and outside of it at will.
Sometimes all it takes for a poly situation to go wrong is for new partner to be mono at heart or worse introduced from outside of the original group as a "friend" (someone they have been fucking behind the groups back on the sly being the absolute worse case) of one of the pre-existing partners and then have one or more partners agree to the addition but only to not make the dominat partner uncomfortable because they have such strong feelings for him etc.