Besides legalities, what are the similarities and differences between "open marriage" and polygamy?

Generally I define polyamory as a close loving relationship between more than two people. I tend to see relationships like that as involving a trio (or more) who live together in an equal partnership between all participants. Such a relationship is usually still exclusive, but exclusive within the group.

Polygamy is one person having several spouses (legal or otherwise). It doesn’t necessarily mean that he relationships are equal or go in all directions. For instance a man migh have two wives but the wives might not have a relationship between themselves: they might not even know each other. The person who has several spouse generally expects the spouses to be exclusive.

An “open” marriage is just one in which the married couple give each other consent to do what everyone else calls cheating. There doesn’t have to be actual relationships involved at all.

I think whether it works or not depends on whether the people involved really trust each other to abide by whatever “house rules” have been agreed, and on how they handle jealousy/insecurity. For some it works. For others it doesn’t.

I think polygamy is a form of open marriage but open marriage doesn’t at all necessitate polygamy. I know of polyamorous relationships where a person has two partners that, if they could, they would marry and both would be their spouses (and I, myself, was in a relationship like that some years ago where I had two equally serious relationships with two people) and I also know where there are primary partners and other partners that you date/see/have sex with on occasion (which is our arrangement now) and there are variations throughout.

The real in that deal is the actual mechanics. In polygamy the people in the union are all committed to their union, be it 2 wives or many more. The husband only has relations with the wives, and the wives with the husband only, no outsiders. Legally since you can’t have a legal polygamous marriage it would appear to be an ”open marriage” with live-in women.

An open marriage can be between just two people straight or Gay. There is a legal spouse but both have signed on to the fact that the other can date and have sex with other people –maybe with preapproval or some other qualifier—, but they are not committed the relationship devlod of outsiders.

That would be the difference.

Polygamy requires the women to be true to one man. There is not sex outside the family by the women of the man (husband) who heads it.
Open marriage sex is “allowed” outside the legal bonds by the partners.

Polygamy means having more than one spouse but, practically-speaking, polygamists are almost always polygynists: one husband, multiple wives. There’s an obvious inherent sex-based power inequality in any heterosexual polygamy; in polygyny, that power inequality is patriarchal. This is a problem in-and-of itself considering the greater patriarchal society we live in.

Now, I’m not saying there can’t be loving, respectful, fulfilling polygamous marriages; I’m just saying the inequality is there. But inequality isn’t necessarily bad. In spite of current lingo, most people in the world do support and willingly submit to some inequality.

The U.S. President, for instance, has more power than your average U.S. citizen. Even if you decry the President’s vast wealth, still, someone has to make the decisions and guide the practice, right? In a polygynous marriage, that someone is the husband.

The problem with polygamy is that the inequality is not based on individual merit but, rather, sex. That’s a big problem becuase, aside from individuality, there’s no good reason to think any given woman has less intelligence, less physical ability, less spiritual authority, less practicality of mind, than any given man. In situations like this, people become defined by an aspect of themselves they are born into and can’t change; they lose some flexibility in life, some choices are denied them; and if the fit isn’t right, they suffer and those around them suffer. This is true of both sexes. Does a man who, by character, is not a leader do a good job of leading? Does he enjoy his role? Do his ‘followers’ enjoy their roles and do a good of ‘following’? No, no, and no. The whole system collapses.

Open marriages, on the other hand, can take many forms. The only thing that defines “open marriage” to everyone is what it isn’t: not monogamy. When it comes to specifics, open marriages are defined according to the parties involved. They can be equal – both partners exercising the same rights, engaging in the same practices – or they can be unequal. Assuming both parties are being honest and sincere with themselves and each other, there’s no harm, no foul, either way (equal or unequal). That’s because the arrangement has been made based on the individual characteristics, desires, morals, of the people involved.

Positive and negative effects? With polygamy, it comes down to luck: whether or not the people involved fit their sex-based roles. With open marriages, it comes down to one’s insight into oneself and communication skills. Both systems can work out wonderfully; both can be terrible disasters. Frankly, I’d rather not rely on luck, but that’s just me.