Yay! This is my 1000th post! 
You know what? I think she just doesn't like the negative connotations of the label "swinger". Call it something else. Or call it nothing at all. Just call it having fun. And if she says, "Oh, we can't do
that..." you say, "Says who? Who else are we married to, here? I'm okay with you
[blankity-blank-blank]ing Bill if you like. I think that's really hot. I just want to see you enjoying yourself."
I still inwardly cringe at the term "swinger" myself. Draws up nasty images of dinner-plate sized gold medallions, leopard-print purple spandex encasing bodies that are not designed to wear it, and enough body hair to keep a sasquatch warm. It just ain't the truth, but it's ingrained there. If she's thinking Austin Powers, don't use the word swinger.