The following is an answer and/or comment by inhahe aka ColorStorm (inhahe.com - myriachromat.wordpress.com).
Of course, especially in a country that boasts religious freedom and neutrality. I'm not even sure why anybody would say it shouldn't.

There's no rule-book of life that says that monogamy is better than polyamory, it's just what humans have chosen to do.. and it's largely out of jealousy, control, possessiveness, and ego. People just can't stand seeing their lover with someone else, so they put restrictions on their activities regarding whom they share certain kinds of intimate joy or pleasure with. Should true love really put selfish restrictions on the other from doing what makes them happy? So it's because of humanity's immaturity that we have serial monogamy.

Sure, some people just prefer monogamy anyway for reasons beside possessiveness or whatever, but even in that case, why should *your* preference be enforced on other people? People just reject the idea of legalizing polygamy is for irrational, conservative, reactionary reasons. They just want to enforce and maintain the status quo. They're afraid of the culture being replaced by something they're not used to. But that's no valid reason to hinder the means by which other people are allowed to express their love.

Not that I believe in marriage anyway (for reasons explained here: https://retrospring.net/ColorStorm/a/942548 ), but as long as people are getting married we might as well let them marry how they choose.