Hey, Mmmm,
This is always a difficult question and it gets asked a lot. We never want to upset our spouse and always think they might "get the wrong idea" or be hurt just by the fact that we asked the question. That's always a chance you'll have to take, especially when it's a question y'all have never discussed before. So you have to make the decision of whether or not to ask. Is it likely she'll divorce you just because you asked? Probably not, after twenty years.
Here's what we suggest: Sometime when it is quiet, the kids are out of the house, and maybe y'all are cuddling on the couch, say, "Tell me, Darling, How do you feel about swinging?"
If she says, "I've heard some couples do that," you'll know she's at least thought about it and is willing to talk.
If she says, "Oh, it's a lot of fun! Do you remember when we used to take the kids to the park when they were young?" you may have to say, "No, I've heard there are couples who switch partners with other couples and have sex, just for fun. You remember? Like in college? When we were hippies? It wasn't making love, it was just sex for fun. What do you think of couples who do that"
Anyway, you get the idea... The only way to ask is to ask, but don't ask a "yes" or "no" question. Ask for an opinion she'll have to think about and discuss. If she gets really pissed, have forty bucks or so ready for a dozen roses the next day.
But don't be surprized if she brings the subject up the next time.
Alura