Even though the marriages on TV are very different from marriages in real life, there are some similarities that are present, and these similarities are what tie the "non-realistic" to the "realistic," and make these kinds of shows popular and appealing.
Marriage in this type of show specifically, is almost portrayed as a joke. It seems that most every character in the sitcom has been with one another more than just once, and having a marriage that lasts is rarely seen. Once a couple does decide to get married the drama begins, and does not stop until the wedding; you have bitter ex's trying to get their old flings back, parents who do not agree with the marriage, brothers and sisters fighting over the soon to be spouse, the couple themselves fighting due to the old flings that appear, and most dramatic, the ones who come forward about the cheating or dishonesty on the day of the wedding.
Beyond the obvious soap opera cliché, if a marriage does continue on, it is an extremely sweet one, the characters seem to be in love, and the show makes a connection to the people who watch it. Most every person longs for love, and if they have not found it yet, there is a sense of completion, in their eyes once they see the commitment made. It seems that the couple's love for one another will go on forever, but the truth of the matter, is that it is a TV show and the audience wants to see drama.
Since I would assume, most people have not slept with just about everyone in town, or have not married their ex's father or mother, or their ex's brother or sister, and the ways of deception just go on and on, especially in the TV world. When the audience sees this, they see the couple not lasting for more than a month, they see them trying to hurt each other, and in the end, they break off their "perfect match."" In doing so, this fulfills the audience's need for deception, betrayal, and deceit but, still gives them a since of stability, and most of all, keeps the audience's attention which is exactly what the producers want.