I too agree with the PP that it seems you are looking more for a sexual arrangement than a romantic one... ie. swinging.
To be honest, it depends on the type of person your wife is how well she will take the discussion. And only you will know that. Personally, based on your reasons, I don't imagine any way of bringing it up will be painless. She will probably feel insulted, for many different reasons. And it could be very negative.