I sent the link to this forum post to my wife, and she had this to say...
"Be blunt. That's what I did, how else would I have done it?"
It's true, she did. It was very blunt, but she had already dealt with the possibility that I would have said, NO. That is what you need to think. I would just pose the question with a serious look in your eyes and be prepared for the oncoming debate. Yes, there will be debate. However, at that point its not going to be a short conversation and it shouldn't be resolved easily. For the first part of the conversation, don't concentrate on the who, what, and where... work on the why.
Why do you want to swing? Why, are you unhappy? Why, are you bored?
Because WHY? It will be FUN!
Take your time, discuss the options and maybe get aroused by the idea. That is the result you will be looking for.
Now, if you prefer the subtle approach, then by all means go for it. However, once your husband gets your drift, he might be insulted or even suspicious of something else. Jealousy and honesty are tied tightly together, so I see the honest route as the best route to take.
Good Luck!
OverKnight