Yes I understand what you are saying I am not rigid in the sense that a person should not have changing needs or desires. Of course if xyz changes at any point it should be brought up and renegotiated. That's the whole point. That's what prevents cheating in the first place.
It's really not all that difficult to say to your partner, I need.... I feel... Before you get so pent up with resentment that you do things to permanently damage the person you love.
It's also not that hard to pay attention and listen to what they are saying they need if they have the courage to do so and try to collectively and creatively come up with a solution. If that solution doesn't satisfy keep trying. If you don't get some satisfaction or enjoyment out of that type of honest interaction then why be in a relationship? Get a roommate.
Sometimes what people think they want or need (sex or affection outside their primary relationship, with or without their primary partners consent) does not fulfill the emptiness they are trying to fill, as evidenced by all the drama and unhappiness on here, on infidelity forums, on marriage forums, umm pretty much anywhere.
If I didn't like sex so damn much I swear I would shave my head and become a Buddhist nun.