I’d have to say that polyamory is somewhat more complex legally than same-sex marriage generally. The countries that do practice it tend to only allow multiple wives, and generally hugely disadvantage all the women in the union as a matter of course.
That being said, there’s no intrinsic reason not to consider polyamory, except that it is a much grander societal change. I suspect that marriage is a step too far, given that most countries do not even recognize polyamorous relationships in any legal way — the first step would be some sort of basic legal recognition. I also think that a major issue is the very low percentage of the population in a long-term polyamorous relationship that is equivalent to marriage — lifelong unions and polyamory are not always compatible to marriage, especially in a legal sense.