Is swinging cheating?
No. Cheating is breaking your relationship's agreements behind your partner's back. Swinging is the opposite — both partners knowingly agree to recreational sexual experiences with others. The consent is exactly what makes it not cheating. A swinger can still cheat, by breaking the agreements they set.
Cheating is defined by deception and broken agreements, not by the number of people involved. Swinging is a form of consensual non-monogamy: both partners know about it, both have agreed to it, and the activity happens inside boundaries they set together. That mutual, informed consent is precisely the thing that distinguishes it from an affair. The presence of another person isn't what makes something cheating — the lie is.
It's worth being clear that swingers absolutely can cheat. If a couple agrees to play only together, in the same room, and one partner meets someone privately and hides it, that's cheating — they broke the agreement and concealed it. Non-monogamy doesn't remove the possibility of betrayal; it just relocates the line to wherever the couple actually drew it.
The thing that trips people up is the assumption that monogamy is the only honest default and everything else is a step toward dishonesty. In practice the honesty is in the agreement-keeping. A swinging couple who talk openly, set clear boundaries, and keep them are behaving more transparently than a monogamous couple where one partner is secretly straying.