Do The Hebrew Scriptures condone Polygamy as much as we assume?

Before I get into what the Bible says here. I want to remind everyone that I'm a Libertarian in my political views (at least Socially). So regardless of what Marriage should be Biblically, Spiritually, Morally or Ethically I absolutely do NOT want the state regulating it in any way. I would rather marriage not be a civil institution at all, but as long as it is I fully support legalizing Gay Marriage, Polygamy, Bigamy, Polygyny, Polyandry, Polyamory, Groups Marriages, open marriages and so on.  As long as all parties involved are consenting adults it's none of my or the state's business.  Now to get to the subject at hand.

Does The Bible define Marriage as "Between one man and one woman" as so many of my fellow Fundamentalists say?

First off marriage is an English word. Ancient Israel did have an institution of a union between a man and a woman, and the New Testament uses that relationship symbolically a great deal, (The Church is the Bride and Christ the Bridegroom, but on our wedding night it will be the Groom's Blood presented on the sheets). The gender designation of the parties in that union is specific.  In fact in Hebrew the word for woman is also the word for wife, and the word for man (as in the adult gender distinction, not as in mankind) is also the word for husband. But the modern custom of marriage practiced by western Christianity has numerous differences in function and practice from that union, well before we debate the gender distinction or number of people involved. I've written a dissertation on the verses alleged to condemn homosexuality altogether, they don't.

It is popular to point out that Polygamy was practiced in the Old Testament. We casually use polygamy to refer to a very specific form of polygamy, the word strictly speaking covers any kind of marriage with more then two parties involved. The polygamy practiced in Ancient Israel was Polygyny (One man with plural wives) and even a specific form of that, Harem Polygyny/Concubinism, which involved each Wife/Concubine having children of their own (or trying to anyway) and raising their own children, creating basically mini-families within the family. This can create rivalries for the favor of the father, and increase the chance of incest, if the children by different mothers aren't entirely raised together the natural barrier to being attracted to one might not develop as it normally would.

To call it common is simply misleading, like in most polygamous cultures it was mainly the wealthy and upper-classes actually practicing it. The Torah never outright forbids it in general, but Deuteronomy 17:17 in laying out guidelines for the future Kings says "Neither shall he multiply wives to himself, that his heart turn not away: neither shall he greatly multiply to himself silver and gold." They failed to obey this (Some anyway, at-least the first three or four, and especially Solomon) but it's interesting that the individual most capable of supporting that lifestyle is the one explicitly advised against it.

The Law of Moses tolerates many things that are still morally disapproved of.  There are no actual laws in the Exodus or Deuteronomy law codes against Drinking/Drunkenness or Prostitution either, but endless verses making clear they're disapproved of morally.

God only made one wife for Adam (Lilith is extra-Biblical nonsense, drawn on misunderstanding Genesis 1 and 2 and how they relate to each other). He could have made many more just as easily as he made Havah(Eve) but he didn't.

The first account of polygyny being practiced is among Cain's descendants in Genesis 4, now it would be unfair to condemn them based solely on that descent, but it is possibly a revealing context. The Midrash interprets the names as an attack on polygamy. Adah is there interpreted as the deposed one, implying that Lamech spurned her in favor of Zillah, whose own name is understood to mean she shaded herself [from Zillah at Lamech's side].

When it is practiced among the protagonists of the Biblical narrative, the results are jealous rivalries between the wives and their offspring. Abraham and Jacob were both forced into their situations by circumstances more or less beyond their control, they were not very extreme examples and yet those problems still occurred, and are the root of the Middle East crisis we have today (Arabs being mostly derived from Ishmael). Later Leviticus 18:18 does specifically condemn the kind Jacob engaged in, marrying two sisters.

I've seen some pro-polygyny sites make a point of Miriam being punished for disapproving of Moses's marriage to a "Cushite woman". I view the point being a condemnation of racism, Miriam was objecting to marriage to a foreigner, not a second wife. And besides, scholars don't even agree that this was a separate wife form Zipporah (We don't know the ultimate Genesis 10 background of the Kenites for certain, and there were Cushites in Arabia). And some would interpret that it didn't matter if Moses was in the wrong or not, Miriam was still out of line, I don't like that way of looking at it though. I've seen websites even cite Esau's polygyny as an endorsement of it, I don't like to vilify Esau as much as Jewish tradition does, but how he ran his family was certainly viewed critically by the Author of Genesis.

It's clearly a source of problems in David's family, the Bathsheba incident was the finale trigger, but it drew on roots already there, from seeds planted by David's polygyny. And with Solomon it's firmly linked to his inevitable corruption and falling away into idolatry, though their mostly being foreign, (point being religiously foreign not ethnically) is also a factor. Solomon didn't follow the other guidelines given to the Kings in Deuteronomy 17 either, which means the connection to that chapter isn't a coincidence.