In human history there have been various models for marriage. A very common one in the Middle East, Africa, and Asia was polygyny, with prosperous men having multiple wives. A tiny number had the opposite, polyandry, women with more than one husband. Some cultures allowed for a formal non-marriage, as when men took concubines; these women had less status and rights than wives but they were still "respectable."
Religion has often influenced marriage, as when the Koran specifically allow men to take up to four wives. In the other direction, the Shakers accepted married couples as converts, but forbade any sex, at all, ever (no surprise they aren't still around).
In science fiction, authors have posited new twists on marriage. One of the most famous is the line marriages in Heinlein's The Moon Is a Harsh Mistress (one of his best books, in my opinion). In Heinlein's lunar civilization, there were many more men than women, and life was so uncertain that families formed in an unending line. Three women might be married to seven men; each woman counted each man as her husband and vice versa. When someone got old or died, he or she was replaced by a new spouse. This didn't stop the married protagonist from falling in love with a woman; it just meant he didn't have to get divorced to marry her.
Likewise some stories suggest marriage might be abolished (although in dystopias like Brave New World this is a clearly bad thing). But for the most part, marriage in some form seems to have staying power. It might be easier to do, harder to do, have a time limit, or not have the same legal implications, but it's still around in a lot of futuristic fiction. Possibly that's simply a case of authors keeping the story at a cultural comfort level. Or possibly, we see marriage in such a positive light that it's easy to think it will survive. The struggle to legalize same-sex marriage certainly supports the idea that marriage in and of itself has value, that is in fact, a basic human right.
All this is on my mind today because 29 years ago on this date I got married. I am still married to the same husband— who, by the way, married me not knowing I had a deep-seated need to write spec fic novels. It came as a shock to both of us, but he's still around, and he even goes to conventions with me. Our "anniversary trip" this year will be to the World Fantasy Convention in San Jose.
I might write fiction about other options, but in real life, monogamy works for me.