East, respectfully, I see your point. I would agree that swingers are not *unfaithful: not adhering to vows, allegiance, or duty.* Swingers should have talked it out, agreed to their own guidelines. Even if they promised fidelity at their marriage, they've decided to modify their vows.
But *infidelity* has a specific meaning, a *definition*. The activities that swingers engage in are contained in the definition of the word; i.e., when swingers play (assuming they're married,) infidelity occurs. I'm certainly not saying that's a bad thing - in fact I think it can be a very good thing.
And while, yes, sometimes definitions change, I doubt the definition of infidelity has changed in decades, and I don't see Merriam-Webster rewriting it anytime soon.
Words do matter.