Interesting thought, Q...
Your current dilema has always bothered me as well... I know this is "only" cyber space; nevertheless, some pretty strong bonds are formed here. So, when I see Dominants (submissives as well) in multiple relationships, I feel someone is being cheated in that relationship.
It's difficult, i suppose, for one person to fulfill all your needs... To "complete you" - to quote a line from Jerry McGuire. Still and all... I feel very much like you do. Either commit to one person ...or dammit .... play the field.
Of course, i've always been a one man woman... totally devoted and completely loyal. If Master Rob were to tell me that he's taking another submissive... although I would be heartbroken ...I'd have to say "well have fun and adios." I expect 100% and I don't believe you can have 100%, if you are involved in more than one relationship.
However, if it is agreed upon at the onset perhaps that makes a difference.
I too am very interested in the responses of Dominants and subs who exist in multiple relationships. Thanks for starting this thread, Q.