Basic Trope: The point at which a relationship gets boring.
Straight: Alice and Bob have been married for years, but they never go out anymore, and haven't had sex in a long time. They have gotten into a routine, which has taken a toll on their relationship and attractiveness to one another. If this keeps up, it could lead to an affair, a divorce, or at the very least dissatisfaction and boredom.
Exaggerated: Alice and Bob are so bored with each other, and find one another so unattractive, they can barely stand to look at one another. May or may not end in a Toilet Seat Divorce
Downplayed: Alice and Bob have been married for years, and have fallen into a routine. They have date night on Friday and sex after every single week. They don't mind farting in front of each other. Alice lets Bob see her armpit hair; Bob lets Alice see his back hair. They're content although it's not exciting, but their relationship is pretty boring to outsiders.
Alice and Bob's "first flush" and newlywed bliss phases have long since worn off, and they have settled into a routine in which mundane responsibilities (child-rearing, work, housework, etc.) take first priority.
Alice and Bob married for reasons other than love, and did not grow to love one another over time.
Bob and Alice's relationship has only become stronger over time.
Alice and Bob were foisted off on each other in an Arranged Marriage and have since fallen madly in love.
Alice and Bob have been married for 50 years, and their relationship is still exciting and wonderful, more even than the time they got married.
Subverted: Alice and Bob take a trip to Jamaica and leave the kids with Grandma for a week, hoping to bring the romance back, which they do.
Double Subverted: But once the vacation is over and they get back into the routine, the romance is reduced greatly.
Alice and Bob now sleep in separate beds, and Bob snarks about how unattractive Alice is getting (and vice versa).
After Alice and Bob return from their honeymoon, the sappy music abruptly shuts off, and the shiawase bubbles in the background pop.
When Alice is asked about her marriage with bob she asks "Who?", even though they are standing next to each other.
Zig Zagged: Sometimes Alice and Bob are bored, sometimes they're not.
Averted: Alice and Bob's marriage does not become stale.
Enforced: The writer doesn't believe in marriage, on the grounds that it gets boring.
Lampshaded: "We've been married so long, I'm starting to get sick of it!"
Alice and Bob get married. They move to Suburbia, and they get jobs that are high-paying, but take up lots of time and energy. Then come Babies Ever After, which Alice and Bob are constantly taking care of. Before long, they have no time for one another.
Alternatively, Alice and Bob spend every waking moment together, but they get annoyed and bored.
Exploited: Claire, Bob's high school sweetheart turned Stalker with a Crush, lets him marry Alice and bides her time until he gets bored of Alice...
Defied: Alice and Bob resolve to keep things interesting by going out at least once a week and trying out new things both in and out of the bedroom. They adjust the amount of time they spend together: not too much or too little. They keep up their looks as much as possible, and they manage the time they spend on responsibilities (or, if possible, hire people who can help, such as a maid, or a live-in nanny), so that they have time for one another.
Discussed: "Our relationship has become like a bad movie."
Conversed: "Hey, Carol! Want to bet on the time it takes for Bob and Alice from Long Runner #1 to get sick and tired about their marriage?"
Alice and Bob's bale their boring marriage on force of habit, but it was caused by entirely different factors.
Alice has become too occupied with work and Bon has entered the marriage with the wrong expectations. Over time this takes a toll on their relationship.