NationStates

It is regulated, but generally legal. We have strict regulations because while polyamory is seen as a mostly harmless practice, some forms are patriarchal and detrimental to the great women of our nation.

Illegal.

Is polygyny legal in your nation?: Yes (men only).
What about polyandry?: No. Polyandry by either sex is illegal due to concerns about HIV/Aids, concerns about weakening of male military morale and (in the case of female polyandry) religious and cultural beliefs about the value of female monogamy.
Same-sex polygamy?: No. Monogamous same-sex marriage is legal though following consultation with Ludovic-Mohamed Zahed
Also maybe explain how your culture views polygamy?:
Polygamy between a man and up to four women is accepted (provided he treats them women well and equally, caring for their financial and emotional needs) culturally.

In our nation, polygamy is legal and protected by law. It is somewhat frowned upon by some aspects of society, while other parts are indifferent. As a Christian nation, polygamy is uncommon, but marriage licenses are still granted to polygamous couples, and it is illegal to discriminate against someone in a polygamous relationship.

Polygamy is banned in the Common Network of Awakened Cliques because it is seen as a form of corporeal selfishness. People who engage in polygamy are publicly ridiculed, humiliated and then executed in front of a crowd. The ruling superhardliners have no respect for human beings and treat them as robots with no personality or humanity whatsoever. Thus the punishment for polygamy is extremely-draconian and violates all known laws of humanity.

Polygamy is often practiced in the Wastelands, although it is Always 1 male and multiple women,
as the types of men who would share one woman where all killed in the aftermath of the Apocalypses or failed to reproduce in the 100 years that had passed.

Polygamy is still banned in the Enlightened State. Despite calls of the Libertine factions to legalize it, alongside other sexual acts that are currently banned, the Enlightened State is sometimes conservative in sex and reproduction policies, and as such, things like polygamy and abortion are still illegal or heavily regulated. The government does have some liberal stances on sex though, since all schools, private or public, have to teach about sex and reproduction.

Polygamy is likely banned due to the fact that polygamous societies are more likely to be unstable, violent, and unequal, things that the government tries to avoid, and not because of religious dogma.

Is polygyny legal in your nation? What about polyandry? same-sex polygamy?

1- Yes. It is legal
2- No. Polyandry marriage are not recognised by government
3- No. It is illegal

"Is polygyny legal in your nation? What about polyandry? same-sex polygamy? Also maybe explain how your culture views polygamy and the history of it?"

Polygamy or polyandry, by all sorts, regardless of the sexuality of the parties, is illegal since 1605.

Polygamy has been a huge taboo in Hellenic Kingdom since the introduction of Christian to the lands.

all 100 percent illegal

Depends on which religion you are. If it allows, it’s legal for you, if not then no. This is only applicable for the recognised religions, if you are from a non-recognised religion then you follow secular law which atheists vote for, 100% illegal.

Polygamy is not illegal, however, it is not necessarily recognized as a legal marriage by the government. Churches may issue non-binding marriage certificates between multiple people, but government offices are restricted to issuing marriage certificates between two consenting adults only.

Some legislation has been introduced to change this, but has never passed.