is it "wrong"? - i think that's up to the individuals involved to decide
Exactly - and thats all there is to it. In my view, its a matter of individual perspective. Its no one elses business to decide on how two people can relate to one another or with how many - its not an issue of morality, its an issue of individuality and of consent.
If there are no secrets, no mistrust and everything is out in the open - then there is no harm - but so many people do it in secret - and if they do it in secret, then there is an issue with what is accecptable and not acceptable within the confines of the relationship.
I am more traditional in my way of thinking though - if I am with someone, who I care about, someone who I have developed a bond with, then I dont share - I see it as a sign of respect and love toward who it is that i am invloved with. I dont need to be the end all and be all of someones world though, that is unhealthy and its unfair.... but if I am capable of sharing my innermost thoughts, developing trust, friendship and sincere desire to protect and care for one other human being... then call me old fashioned i guess.
Personally - I dont believe you can get to that level of love and depth with more than one person, (but I am not saying that you cant develop any kind of depth either) but perhaps thats just me.....its hard enough just getting one person to be interested in you at all let alone many.