Maybe it's my conservative nature, but I'm surprised at the support and responses in here. Then again, you're probably talking to teenage males who think it would be a good idea. There are many problems that will arise if it's legal and done. In no particular order...
1 - Jealousy. What's gonna happen when a guy treats one woman as his favorite? The other 'wives' aren't gonna like that. Some might even lash out against the favorite. Let's face it, in this culture, women don't even like it when there husband is seeing even 1 other woman.
2 - Intermarriage. Having kids with say, 50 wives depletes the gene pool (same guy). So, when these descendants meet other descendants from a polygamic family, and have kids, that might be a few thousand "family". If this population gets large enough, they have no choice to intermarry (whether by choice or by accident). Such a thing could lead to disease across a broad area of the population. (i.e., the origanal male had a genetic disease).
3. Moral - You knew I was gonna mention this one sooner or later.

Or rather, this would lead to a slippery slope of a decline of morals. (granted, other cultures do it, but it doesn't mean it's always right). Marriage should be a union for life between 1 man and 1 woman. (don't go off topic in off topic on this one.

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4. Living conditions - How will such a family (and a large one at that) live? If they're a family, then they'll need a house to live in together. A typical house (2-3 bedrooms, 1-2 bathrooms, livingroom, kitchen, basement) just isn't going to cut it, unless you want one huge slumber party in the basement. Just imagine getting ready in the morning. Even if they had a huge house, or mansion, you'll need to pool together resources, and even that can lead to problems (i.e., one saying they paid more than the rest, so they deserve more), which leads to the next point.
5. Legal - What happens if the man decides to devorce 1 or more of his wives? Or, what if he dies? You're gonna have multiple people fighting over who gets what.