"Marriage" is a religious construct that typically defines itself as the union between one man and one woman.
So I'd say that anyone, male or female, can only have one spouse, by definition.
However, as long as they don't get all hung up on semantics, I think it's perfectly fine for one man to co-habitate with many women, or one woman with many men, or several men with several women ... as long as there are no children involved.
I added that last bit because children have a hard enough time learning what it takes to get along in this world without needing to wrap their little heads around a highly unusual domestic situation. ("Who's your mommy?")
Of course, this is a very broad statement designed to accomodate the generic nature of the question. If you could point out a bunch of people who are engaging in this type of practice, then we could take a look at the individual circumstances and render our completely irrelevant judgement on them based on a more detailed profile.
Being judgemental is fun!!
