I'm not sure how someone having sexual threesomes would have anything to do with poly marriage...
From what I've seen on these forums, a lot of poly folk cohabit with more than one of their partners. Some have had commitment ceremonies with the partners they can't legally marry. They're already essentially living as married groupings; it just isn't legal, and as such has no protection under the law.
I've also seen--even just this past week or so--people on here posting about being threatened with losing kids, homes, and jobs, or even actually losing them, because of being in polyamorous arrangements. Legalizing poly marriage would hopefully eliminate this type of discrimination, assuming that the legalization of the marriage included, or a separate law was passed, the illegalization of discrimination based on living arrangements.
It's interesting to see people's take on this here; a similar discussion is going on in a writer's forum I belong to, where I and two others are trying to explain what polyamory actually is and that yes, there is a lot of discrimination, while the majority, who aren't poly, either struggle to comprehend or tell us we're lying about the discrimination...