The temptation to cheat is there for everybody, regardless of how likely it is to happen. You might be 100 % certain in your own mind, that your partner would never cheat on you. But nothing is ever 100 % certain, there's always a possibility, however unlikely, that any given situation can happen. That doesn't make it a forgone conclusion, just a possibility
Cheaters only become cheaters, when they've actually cheated. Until then, they're just potential cheaters, and every single person in a relationship is in that category. Anybody who claims, there is no possibility of them cheating ever, in any circumstance, is likely lying to themselves. And that's especially so in an open relationship, where the people involved are constantly having inimate relations, with people other than their partners, and thus have a much greater probability of meeting that one person who would cause them to betray their existing partner. Because nobody is immune to temptation. Nobody is perfect enough, not to feel the pull of another, however rock solid they believe their relationship to be.
And if you're in an open relationship, which allows you to take things much further, than an ordinary person, without any comeback. The temptation to take it even further, perhaps overstepping the red lines that you've agreed, can be that much stronger. Because people in that situation, are much more likely to believe they can get away with it.
Now I'm not suggesting for a moment, that people in open relationships generally follow this pattern. I'm sure most of them dont, because as you rightly claim, they've already been given the keys to the kingdom, and most people are going to be satisfied with that. But there are always exceptions to every rule, and claiming that there aren't is naive.
I never said my experience was a bad one. That's what you suggested. I decided not to let it bother me. We we're already in an open relationship, so I decided to discard my red lines. I just never told her that I had. That way, she could carry on believing she was cheating, and get whatever intense thrill she was getting from it. And of course, feeling incredibly guilty everytime. But that's the price you pay.