There are a lot of potential factors to consider. If a sub has limited interests (and no desire to expand) and sees a Mistress frequently this can become an issue. However most devoted subs of this nature aren't looking for something new each visit. Spankos and foot fetishists are good examples of this. For them it's the little things: a new paddle, a new short skirt, RHT stockings, a new pair of shoes or a new role play . . .

If a sub has a variety of interests, then it's almost impossible for a creative Mistress to "run out of ideas". Interests like humiliation, electrical, medical, etc are almost a bottomless well for fun scenarios. If a Mistress enjoys what she does and wants to keep things interesting for both her and the sub, this should not be a problem.

That being said, I do understand the desire to see other Dommes and I have never been upset to find out that a sub has seen someone else - unless he is collared, then he needs to ask permission. Monogamy works in marriages for some, but not for others. I think it is the same in D/s relationships.