Just in the vanilla world, which is where most of my experience lies, I think there is quite a social stigma towards dating multiple partners.
I definately agree with the others in the way that, If i alone am not ghood enough for you, then why am I with you? Of course there are exceptions to said rule, as there are exceptions to all rules, but I am pretty damn possesive.
Secondly, Its more acceptable (seemingly) for a Dom/me to have multiple subs, than a sub to have multiple Dom/mes. Makes sence i guess, but just another thing to think about...
(Shhhhhhh I'm sleep deprived)