Situations like these can be cumbersome and emotionally taxing. A marriage has been a bond of two individuals for ages now, and the concept of open relationships is new to the society. A relationship counseling can be one of the most preferential solution here. Sexual dissatisfaction can be a major reason behind your spouse proposing such a proposition, talking to her over the same lines can help you sort the situation on a subtle note and might even help in preserving your emotional relationship.
However, if your wife still persists on the same, we would suggest that you either try to come to terms with it and maybe even experiment for the sake of it. On the flip side, contemplate on the type of relationship you have with your wife and communicate with her.