Check the archives
here or check
here for lots of threads on the subject.
As Spoo said, you've got a few choices:
1) "swing" without her, which, by the way, is called cheating. You'll risk your marriage and not have too much fun anyway, since most swingers won't play with someone who doesn't have spouse approval.
2) Leave it alone and never bring it up again.
3) In a non-pressured way, talk about fantasies with her. Bring her to this website. Start a dialog. There are a lot of people who started out with one idea about swinging, but changed their mind over time. I don't know your wife, so I don't know how open she would be to it.
Pepper