Ah.. but what if this is a "don't ask, don't tell" situation? Perhaps they have their partner's permission to be out with others, but don't want to "know" about it.
Spent some time wrestling with this one myself during my first long term "poly" relationship. Here are the conclusions I've come to so far:
http://http://polytripod.blogspot.co...dont-tell.html if you'd care to read more on the particular thought processes around that idea.