This question reminds me of that classic Arnie flick - "The Running Man" (Arnie's movies have so much to teach us!).
Arnie was set up for a fall by some crooked cops/militia, convicted on trumped up evidence, and then forced to play a "game" to the death where the odds was stacked against him. He didn't make up the game, and had no input into the rules. However, he did clearly understand the rules.
He survived the game. I'd have to say he broke quite a few rules along the way in order to survive!
I'd say that men do not "agree" to the rules or expectations of their SO, or those of society in general, when they marry. They understand the expectations because public morality is well publicized. They seldom have any conversation with their SO about whether they expect these rules to be followed strictly, because they assume that women will expect them to do so. They hope they can live by them, because they understand that life will be a lot simpler if they can. Most, for one reason or another, find out that they can't live by these expectations. Thus: massageplanet.netites, massageplanet.net, the majority of mankind, and the everlasting sex industry.
Do we call Arnie a "cheater"? Do we think he was justified to cheat? Do we really worry about the definition of "cheating" once we decide on the "morality" of a situation?
The moral of this story, my friends, is that "cheating" only has relevance if you have any respect for the rules in the first place. As a result, the question should be: "Is it always wrong to go to a rub and tug if you are in a relationship at the time?"
As for my personal view, since I don't subscribe to the "temptation, evil, sin, and weakness" explanation for human conduct, I don't think it's wrong unless you perceive that harm will come to yourself or someone else (who doesn't deserve the harm) as a result of it.