The term 'Law of diminishing marginal utility' is actually describing our human nature which is solely focused towards consumable commodity. First time is always intriguing, second time is not and then the time comes where fascination ends, saturation spikes and the same consumable commodity is not desirable at all as it was at first. I've not forgotten this teaching that I learned myself back in my +2 commerce classes. Very practical.

Relationship has it's own course though — it is in fact run by the law of attraction 'like attracts like" believe it or not. A person with negative energy/thought attract same and a person with positive energy/thought attracts positive experiences in life. Sometimes a relationship ends up being a fusion of two opposite thoughts. It fails miserably. It's one of the partners action that elongates the duration of their companionship and that factor when brought to the equation brings a happily married life. I would like to call that factor/action — 'Compromise'. There is no saturation point in happily married life as you've thought of. In fact, the law of diminishing marginal utility is a bad analogy to describe relationship. I'm a witness of many relationships, decades later their bond and love growing more than ever. I agree there are certain phases in relationships where desperation,boredom comes out of expectation not being fulfilled but compromise and understanding are what gives the continuation to it.