Straight: Charlie and Diana are Happily Married, and have a fairly stable relationship, in comparison to AliceandBob, who are only on-again-off-again dating, and have a Slap-Slap-Kiss relationship. Charlie and Diana are supporting characters, Alice and Bob are the leads.
Exaggerated: Charlie and Diana had Love at First Sight, have been married for twenty years, and never fight, ever. Alice and Bob are only Friends with Benefits, and they fight every day, and when they do, it's like World War III.
Downplayed: Charlie and Diana get into less fights than Alice and Bob, but are by no means a perfect couple.
Charlie and Diana's personalities compliment each other, while Alice and Bob's clash.
Charlie and Diana have been married for a long time, so they know what they're doing, unlike Alice and Bob, who have only been together a few months.
Inverted: The main characters have a more stable relationship than the supporting characters.
Parodied: Alice and Bob are not suited for each other at all, but they still try to be together. Charlie and Diana are perfect for each other in every way.
Zig Zagged: Eudora claims that Charlie and Diana are not as happy as they seem... but everyone knows she's morbidly obsessed with Charlie so the audience and characters are inclined to distrust her opinion... but it turns out she's actually on to something. But evidence from other sources shows that she's not...
Averted: The secondary couple have just as many (or just as few) problems as the lead.
Enforced: "Alice and Bob's relationship is fun, but we need some sanity in this show!"
Alice: Why can't you be more like Diana's husband Charlie?! He would never do this! He doesn't piss Diana off! They don't fight like this!
Bob: Well why don't you act like Diana!
Invoked: Charlie and Diana want to set a good example for Alice and Bob's kids as to what a normal and healthy relationship is like.
Discussed: "Have you ever noticed that Charlie and Diana don't fight at all? Bob and Alice— yikes!"
Conversed: "There's always one normal, happy, sane couple in every sitcom, isn't there?"