I'm a fan of both, honestly. I lean more Star Wars then Trek, mostly because Trek is too clean and the people too perfect. That said, DS9 was probably my favorite Trek show because it felt more real then the others.
That said, in both cases you only see a small fraction of the universes, in shows, movies, or expanded universes. There should be no reason poly relationships can't exist in either setting. Personally I could care less about how much I see. Like my life, unless it's important, does anyone really need to know? Does it move the story along, or is it just a quirk to make a character/race seem more unusual? I'm okay with it being so far in the back we never see it because it's not important. Like how Babylon 5 handled the same sex relationship that cropped up. You know it was a thing but it's only brought up when it's important to the plot (which was all of twice) and handled tastefully.