There is an old saying, "Softly softly catchee monkey".

If you really don't want a knee so hard in your groin that you'll have a second pair of tonsils, do a lot of prep work, leaving your wife lots of avenues of saying no.

Things like going out to dinner as a foursome, getting plastered and staying in a motel with twin doubles "sorry, last room darling".

Let each couple have sex with their own partner (in the dark if need be) and get her used to the idea of other people being around during sex.

She will either become curious or not.

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Calm, quiet, smooth, devastating