Oh, boy...that's not a good situation.
I'll suggest grabbing a copy of Tristan Taormino's new book "Opening Up" and reading it. It describes nonmonogamy of most every sort and also has good discussion of the situation in which you find yourself. I think reading that would provide you with a good grasp of the issues so that you can then discuss the situation with us cogently.
In the meantime, ask your husband for time to wrap your head around the whole concept. Both of you need to understand what it involves before deciding what to do.