A difficult one this!!! I can only suggest that you persuade her that it is a game/fantasy that you can both play - fantasise with her while making love and explore fantasies on this theme together. Eventually you should be able to interest her in the scene of the two of you seducing another man to make love to her - you both need to be involved in the scenario. I have succeeded with a couple of partners and they have really enjoyed the experience and I have enjoyed watching. We planned the seduction together - MOST important!! - you know the scene - the three of you go to bar/restaurant - wife wearing short skirts, garters and stockings etc - a little teasing and flashing - back home for coffee & drinks - more teasing and flashing while she flirts with both of you - kissing and touching in a joking way - blouse undone and skirts up showing panties and garters etc - inevitably leads to more and more until you watch them fucking before taking your turn. In my experience you both need to plan these scenes together as fantasies that can slowly but surely be turned into reality a step at a time - it most certainly can be done but needspatience and tact - most wives or partners will in the end enjoy the experience but have to be lead very slowly towards the final goal and it has to be a shared experience.
If you are seriously interested I can relate in detail my own personal experiences on this matter - perhaps you could ask your wife to read them!! And the sensation of "sloppy seconds" can be the biggest erotic experience for BOTH of you.
