"Trends" aren't universal. There are always going to be exceptions to any trend. With a human population that's already nearing 7 billion, even an extremely rare behavior is still going to have a fair number of people practicing it. And in Trek's future, the human population will multiply greatly, so that even if the percentage of polyamorists continues to decline (it's always naive to assume that any current trend is bound to continue like some simplistic mathematical curve, but just for the sake of argument), the total number of them might remain substantial.
Not to mention that Trek's humanity will be spreading across multiple planets, allowing multiple opportunities for atypical behavior patterns to resurge. Polyamory has definite advantages to the members of a young, small colonial population (cf. "Up the Long Ladder"). And a lot of colonies would be founded specifically by groups that practiced fringe behaviors that had grown scarce in mainstream humanity (again, "Long Ladder" is an example, as are "The Masterpiece Society," "Journey's End," and various colonies in Trek Lit). So even if you were correct about the trend remaining consistent on Earth, you can't rule out the possibility of polyamory resurging in other human populations elsewhere.
I didn't rule it out. I just don't think it has any particular value as theme to pursue. Other writers are more than welcome to pursue it and I'll likely read those stories of they're by writers I enjoy. I don't actually care. It's inconsequential to me.
However: to say the existing systems of polygamy "hurt no one" is, I think, at best, naive. Women and girls are hurt daily by this practice as it serves patriarchy and reduces them to second-class citizens, at best, in their own homes. The "stigma" is deserved and that is the reason it's illegal. MEN abuse it and always have. The more equality you create amongs the genders, the fewer polygamous marriages will occur. How do we know? 100% of our history says so.
In the Star Trek future (as opposed to an hard scifi future) humans are, for the most part, all equal. The few times we've seen serious modification of human behavior, it was played as an extreme oddity. I think a vast multi-planetary diaspora would not lead to poly anything under most circumstances but rather to termed marriages that would, by contract, be renewed by both parties once the term was up.
Or, y'know, no marriage whatsoever. People would be with and breed with anyone they wished in whatever combination.
"Polyamory" to me, describes my adolescence as, I imagine, it does for most. In any case, in any discussion of biological discrimination (skin color, ethnic, sexual "preference")
philosophical discrimination doesn't get a seat at the table. It's not the same thing. All
ideas must compete for a place. If they don't get it, they lose.
How many nihilists are there? Not too many. Anarchists? Nope.
There aren't even a lot of libertarians. The ideas just won't stick, culturally. That's why I said, "barring accident." It would take MASSIVE transformation of the human population, one we've never seen evidenced in any of the Star Trek lit or canon material to account for this shift in our cultural behavior.