I have a great relationship with my wife. If she wanted to have a semi-open relationship in the bed, I wouldn't feel "belittled" by it. I wouldn't make it a habit to have that kind of relationship.....
IMO that is not wife material and not a normal way to live its Man+Woman not Man+Woman+Woman
I can say this since i was with a Bi chick for almost a year and at first it was interesting but later was awkward.