A dominant can have as many submissives as he/she is willing to provide for and as many who want what he/she has to offer them.

As far as your situation is concerned, I don't think you really did anything "wrong." This isn't junior high. Just because you exchange a few PMs with someone doesn't make you an item. As a matter of fact, playing with someone a few times doesn't amount to much, either. You really don't know where you stand with anyone until you have the relationship talk. Monogamy shouldn't be assumed, especially in an alternative lifestyle.