This is a recipe for disaster. I have actually seen instances where eventually it becomes a huge issue in the marriage. At some point there is a huge chance she points to this as part of the reason she is leaving.
First and foremost no husband should ever want his wife to partake in this lifestyle when she does not really want to. respect gentleman, have it for your wives.
Second no husband should pressure his wife into this. Again respect.
When a couple enters into a non - monogamous lifestyle it has to be something they both want together as a couple, not just to make one person happy. This isn't choosing what color to paint the bedroom, this is a huge step.