Answer from an atheist here...
The institute of marriage has not caught up with the rapid pace of paradigm shifts in how society regards its members (at least here in the U.S., which I am most familiar with) . Women are freed, so to speak, from the ancient vicegrip of arranged marriage and male dominance, and the subsequent changes that have followed in the wake of all these changes has caused divorce rates to skyrocket, among other things. How we look at marriage overrall needs a lot of reflection.
Regarding polygamy, I'll echo the views of others, that as long as everyone consents and is happy with the arrangement, I see no ethical conflict. However, I find the likelihood of this situation arising to be rather unlikely, and the risk of discontent arising later into the relationships to likely be higher than among monogamous couples, thus, I would probably caution against it.
Regarding the person who wondered where atheists get their morality from, this question is readily answered - go to Richard Dawkins' website and ask in the forums, and I'm sure you'll get plenty of responses.
Essentially, atheists get their morals from the same places you do, and I don't think Christians or religious folks have any special claim to morality at all. They assent or disregard the moral precepts of their holy texts with regard to society around them and the norms of the day, and thus I regard religion as simply a codifying force of moral opinions already held, not their source. Furthermore, there is much to say about our innate "moral sense" - evolved altruism and our psychology playing a major role in that, such that our evolution plays a major factor in why we are apparently "moral". Evolution explains our behavior much more thoroughly than religion, which offers commands, not an explanation (for instance, psychology and biology provides good explanations of why we do not ALWAYS do what is "good") ; and it is through reason and philosophy that we can reliably derive effective, practical, provisional moral codes.
That Buddhism, Jainism, Epicureanism, and other nontheistic philosophies with their own moral codes have existed for hundreds and in some cases a few thousand years flies in the face of any claim that atheists "cannot" be moral, which is a patently absurd position. Clearly we are, and what baffles me more is that some theists can't seem to understand how, lacking God, one could be moral. That's a funny idea; atheists certainly aren't unable to understand why theists can be moral.