I cannot speak for anyone but myself, if I were the OP, asking for some time to process, fully grasp what has been going on, and working on the marriage, would seem fair. It just seems like the OP's metamour is kind of pushing too soon for too much, and she is uncomfortable with it. Rightfully so. She may know of the OP, but she does not know her personally.
OP, are you even poly? You mentioned in your intro that you and your DH had an open relationship prior to marriage. I am assuming both of you, but I am not entirely sure. I guess I am wondering why he did not just come to you and say, "Hey. Is it possible for us to open our marriage, or at least discuss it," instead of cheating on and possibly hurting you? Maybe that was too simple.