not that i am ever right....
i think if you give it some time, things might come around again. she may have felt pressured or overwhelmed, or maybe even as if you were "telling" her how and what she would be doing next.
this was a cross roads, and she apparently enjoyed it. doesnt mean that she is ready to share you yet.
in your words, it was done with your consent, logically there for you cant feel cheated on, however you might feel you were cheated out of an opportunity. what if she had not enjoyed her experience? would you have her go through with another encounter to fulfill the "deal"?
its not easy, but try to see it from her side. she has not had the tendencies you have, and may not fully understand how you could love her and love someone else too. this is the biggest hurdle for some people. i know swingers that can have random sexual meetings by the dozens, but the slightest hint of feelings, and things can get ugly. allowing a significant other to be free both physically, and emotionally, is big, and not an easy step to take.
hope things work out for the best for both of you.