We're in a similar boat at this time. I brought the subject up two years ago (with tremendous fear) and was met with a completely non judgmental yet not too encouraging (as far as her desire) reaction. I took this as a positive sign. She said that the idea of same room sex with others (either watching or involved) was not particularly appealing to her. Though she could fathom the idea of closed door swinging with trusted participants. We've been happily married for 15 years. I'm 52 and she is 47 and we've experienced enough life to feel confident that we share similar views about the "meaning" of sex and that it's not necessarily about never-ending devoted love. :rollseyes So I don't think there are hidden agendas.
Since that time, I've broached the subject with progressive frequency with similar results. I've shared some of the introductory threads on this forum with her and I would gauge her reaction as less than intrigued, though still non judgmental. She's definitely in the "I would like (i.e. am willing) to do this for you camp." She even said, "Why don't you make an appointment at an off premise club and we'll meet some people, talk and see what we think." We're in the Los Angeles area and I haven't been able to find an off premise club yet, however, I'm quite reluctant to proceed without a little more outward interest from her.
Sex seemingly is not as high up on her priority list as mine.
She says it was on her mind a lot more when she was younger. And she doesn't quite understand/relate to my interest to see her pleasured by other people about whom we feel good.
So my current tact is to put swinging on the back burner for now, and try to figure a way (non pushy) to help her dial up her sexual awareness. I know she has issues about her weight and I think that plays into her desirability quotient. I'm more comfortable about expressing fantasies than she, and would love to make progress that area. It may not lead to swinging in the end, but will improve our communication and sex lives together.