I think there is sometimes a fine line between poly relationships and having affairs. When there is a veil of secrecy or lack of at least minimal familiarity between all patners it creates the same negative energy as deceit and guilt. You're wife is missing a part of the beauty that can be found in a poly relationship. You are feeling used for a reason that is valid and justified but that can be changed if your wife and her partner learn more about poly relationships and how to create positive energy. Otherwise resentment will set in I believe. Good luck..maybe get her and him to do some reading on here.
Playing the Game of Life with Monopoly rules.
Monogamy might just be in my genes
Poly Events All Over