Is polygamy legal if every party is fine with it?

No. At least not anywhere in the US.

If everyone's okay with it, the "second wife" could simply be a live-in friend. The only thing is she wouldn't have any of the legal benefits of being married.

Although considering the courts are fond of expanding the definition of marriage, it's not inconceivable that a few years from now, cases could start showing up that will expand marriage to include polygamy.

It remains illegal in most countries, including the U.S.

Lots of other things like incest remain illegal as well, even if those involved are "fine" with it.

For polygamy, we have no laws set up to handle all the new scenarios that can occur, such as divorce, child custody, inheritance, etc.

If people want to have a "polygamous" relationship, there's nothing that stops them living together, creating contracts to set up things like inheritance, etc. But they're not allowed to marry and have all the rights that normally accompany formal marriage.

Still it would be illegal.

People can't not agree on ignoring a state law.

What they can legally do is to marry ONE of the woman and live as a trio, or to marry none of them and live as a trio, but he can't not marry both at the same time unless the law is changed.

No, it's not. Rarely are the women free to choose. They are bullied into it or told they will go to hell if they don't do it.

Why not polyandry? One women, multiple men. Or is it only ok to you if it's women?

i am not aware of any rule or law of any country regarding this problem but in Islam you are allowed to marry 4 woman but only in form if your first wife allowed you and you are fulfilling all their needs and rights.

Not in America. It should be between the people who want to get married. Any and all headaches of that arraingement would also be on them too.

In backward countries and backward religions

Well, the definition of "marriage" seems to be quite pliant, so yes

Not in the US.

no