Adultery is cheating on a spouse.Thou shalt not commit adultery kind of covers all sexual sin
Actually, I view it more as taking the love that someone who is your partner gives you, and giving that love to someone else without your partner knowing.
And................as far as "cheating" goes..................if a couple are swingers, and they pair up with another couple, who are also swingers, and they decide to have a night of sex with each other's partner, it's not cheating, because everyone is aware of the situation and is in agreement with it.
Most people want to expand the definition of adultery to cover anything and everything that they are against.
I'd expand the definition from spouse to any kind of long term serious partner Roadrunner.