I think the answer depends on if you mean what marriage should be ideally, or what it should be at present or in the near future. Ideally, I think people should be allowed to marry whoever they feel like. However, that just isn't possible right now, because of two main problems with polygamous marriage. Firstly, most people don't like the sound of it. Public opinion on gay marriage has changed quite a bit recently, and I think that apart from fundamentalist religious types, no-one really has a problem with it. Polygamy is seen very differently, and any attempt to have polygamy legalised will fail, guaranteed. While this isn't necessarily an arguement against polygamy, since the same was true of gay marriage until recently, it is an argument against pushing for it now, since it would likely create a backlash against gays as well.
The other problem, as mentioned already, is that the mechanics of multiple marriages would have to be different from current marriage. An obvious problem with this is that most people don't like change. Simply changing "husband and wife" to "husband and husband" isn't a problem, but try to alter the whole marriage system and people will complain. In addition, the changes really aren't as simple as some people seem to be arguing. The rights afforded each partner, parenting issues, responsibilities and so on can no longer simply be split 50:50. What if one person wants to leave but the others don't? I'm sure all these problems can be worked out, but it is nowhere near as simple as simply altering the gender of a few words.
In addition, there is also the problem of what exacly is considered a polygamous marriage. Is it a group of people all together? Is it A and B both married to C, but not to each other? Is it a close relationship similar to a married couple, or is it a looser one similar to many current polygamous realationships? Can the group change as people enter of leave, or is it fixed at the time of marriage? If you choose just one definition then you will be excluding many other polygamous people, but trying to make a general enough version of marriage to encompass them all will require huge changes, and may not be possible at all if some definitions are mutually exclusive.
Incidentally, I assumed the question was more about what marriage should be in the near future, rather than ideally, so I voted for the second option.