Open marriage is a relationship wherein the husband and wife agree to be each other's primary partners, but each of them is permitted to have sex with anyone else they choose, and each one of those lovers is also permitted to have sex with whomever
they choose, ad nauseum. The term is as opposed to a closed relationship in which the number of participants is defined; depending on the model, either everyone has to be on board before another person is brought into the relationship, or, as is the case with biblical patriarchal polygyny, only one person has that authority -- in most cases this is the husband, but I knew some closed triads and quads in Atlanta back in the 1980s and 1990s for whom the authority figure was one of the women.
These are all forms of polyamory,
@Philip. The term 'polyamory' has in recent times been hijacked by the swingers crowd, and on some dating sites the swingers are positively Bullies Central, speaking as if anyone who shows any discretion about how many lovers get brought into the mix is some kind of barbaric cretin.
I'm noticing on sisterwives.com that only a minority of the participants appear to be following the polygyny model. When reading profiles, it becomes clear that most couples and a rather high percentage of the single females are there for grins and giggles, which makes it a place dominated by those trawling for short-term threesomes. I also suspect based on how many profiles I've looked at so far that those who identify themselves as 'Spiritual' outnumber those who identify themselves as 'Christians' or any of the individualized Christian denominations.
@Amanda's observations are spot on; I think it's possible to find someone there; obviously that's the case, given that we know people here who found each other on sisterwives, but one has to have the patience of Job to stick with the process. (I'm still documenting my experiences there and at Zoosk on the Intro III thread.)