Hi Ray. I think you've already received some great insight, but would like to share my thoughts as well.
I think "lifestyle" is the key word there. Your chosen lifestyle at any given point in time is not the same as your life. Things ebb and flow throughout people's lives, and what was super important to them at one point may not be as some future point for many different reasons. It sounds like what is more important to her is having you in her life, not leading the lifestyle she did several years ago, and even then, that was just a lifestyle at most, maybe even more a hobby.
Swinger couples aren't immune from divorce. We know this first hand from what some very close swinger friends have gone through. Swinging, their hobby/lifestyle, had nothing to do with it. It had been a part of their marriage for many years. Other factors in their lives did have something to do with it, same factors that play into the lives of all married couples. There is so much more to life and marriage than sex. I think a very common misconception is that is not true for swingers, but it totally is. I don't find it odd at all that she would happily give up swinging for the sake of your relationship and never even give it a second thought.