Maybe you should take your question up with Dan Savage (writer of the Savage Love advice column). I know I once enjoyed seing my wife making love with another guy but that was a once-off and we don't let that happen any longer. But you know that casual 3rd partners don't threaten your bond, so if you can find a steady 3rd playmate who enhances your bond rather than weakening it, then who cares what the Joneses think? Corrupt religious practices to control the institution of marriage make monogamy a primary relationship and mostly acceptable in society.
My husband and I started playing together a couple of years ago.
We've had a steady girlfriend that we would have sex with regularly, and a few girls we've met with a couple of times for drinks out and then sex.
Also, make sure the woman likes you better than the husband or she is going to steal you man and home in the long run.
While this is like every mans fantacy I caution you that I agree with the first entry.
Once they move out and are on their own, then you're more free to do what you want without negatively affecting the kids; but I still think that in the long run, it will cause more problems than it would solve.