[Additude]

This sounds pretty much like the normal course of action with a lot of couples. Very few couples just dive in. Most progress slowly, with the woman being the most hesitant, for a multitude of reasons.

What you didn't say in your post was "Why your wife is being hesitant" at this point. I think you don't know totally how to approach this, because your not sure where to start with her.

I suggest a healthy conversation with your wife over the subject, understand what it is she is feeling, why she is feeling it and what your both going to do about it.

I think your goal is to make her comfortable with your feelings on the subject. I'm sure your wife doesn't want to put her marriage at risk and she is probably more concerned about your actions than her's regarding swinging.

Once you know where "she is coming from" you'll better understand your angle of approach.