I'd disagree with peanut0017 slightly. In my opinon relationships are meant to grow and change, and i know that in some cases when we are alone we have trouble finding those core parts of us we have always wanted to express. Sometimes when we have a relationship, we have a foundation and can start to look at our more inner feelings and desires, and move forward with expressing them. More of, feeling more confident on finding oneself and trying to bring those thoughts to reality. . but your relationship was built on a set of standards, grounding. Your wife apparently has taken your sexual relationship to be restricited to you two alone. That's a core part of the foundation of your relationship. It's not so simply changed. You have your position, feeling that you know your way you want to live and express yourself, and feel, but are restrcited from touching with it. It's making you feel shitty at the least. Then theres your wife, your partner. She's just found out her husband wants to sleep with other women, have sex with other couples. Go as far into that as you want, cause you just know in her head when she thinks about it, she'll be going into every detail of it. That's if she even has thought about the details. She could not even get that far and brought out a solid wall to the whole situation, because of the foundation of your relationship, that you are sexual for eachother only, and everytime you present the question or the desire she walls you with a simple no. i donno..
You're asking her to change your relationship. To even remotely do that you need to look at it from her perspective. If you can get her to even look at it past the simple 'no', where she is actually openeing her feelings out to the perspective, .. u really are going to have to prove your love and devotion to her. As far as you know she's never had these feelings, so the whole idea is foriegn to her. She'll not be able to connect the dots and take the same paths to reason you are, and she might fall into alot of jeralously, despair... i mean.. u have to be careful. to u its not a big deal, but to her its a major part of ur relationship. ur moving a bottom card of a house of cards .. tread lightly and hold her hand. U've really got to put the effort forward with her on this one. You're already in serious pain, and im sure this is very much contributing to it.. to not be allowed to be who u are, especially because you do not want to harm a person you love,.. it's twisting and tormenting.. but this situation could simply be igniting other issues that maybe, you need to be addressing first before moving forward.
Sex is sex in the end. i hear you on swinging. One thing i know, and as death71 mentioned, some people can not seperate sex and love. She might not be able to even comprehend it yet.
All i can say is, .. respect her. Take it slowly with this with her.