I'm a proponent of do as you will but harm none. If marrying several people doesn't hurt anyone then why not let them do it? The problem is that human nature doesn't exactly work that way. Someone will always try to get something out of it for themselves. They will eventually think "Right - now I have this harem of wives (or husbands) to do what I want them to do - so... I have slaves." Or you have the flip side. "I was the first wife (husband) so I'm Queen Bee - do my bidding." Human nature rears it's ugly head once more. It's rare to find a marriage between a man and a woman (or two men or two women) that doesn't have the dominant one. The boss if you will. Very rarely is it ever a partnership, which is what marriage should be, all things equal. Equal say in what happens, equal rights and equal time. How can a polygamous relationship do that?
But then again - if you get people together that do genuinely care about one another, love one another and accept each other for who they are - numbers are unimportant.