When I read the Old Testament, I find many passages glorifying men who had multiple wives, and, in fact, glorifying the possession of multiple wives. When I read the New Testament, I find nothing telling me, a man who doesn't seek to become an elder or a deacon, from having multiple wives. I live in a country where polygamy is legal and culturally accepted. Is there a biblical reason that I shouldn't take another woman to be my wife?
The Old Testament does record the fact that having multiple wives was a prevalent practice, but you are incorrect in stating that it glorifies the practice. Actually, the examples given show that the practice created many problems. Abraham's wife, Sarah, and his concubine, Hagar, constantly fought (Genesis 16:4-6; 21:9-10). Jacob had to deal with jealousy and rivalry between two of his wives who were sisters (Genesis 29:32; 30:16-18). In addition, Jacob did not treat his wives equally (Genesis 29:30-31). Solomon's wives led him away from serving God (I Kings 11:1-6).
Mankind often practices things of which God disapproves. God regulates the practices, but it would be a mistake to say that God approves of them. For example, Malachi states, "For the LORD God of Israel says that He hates divorce, for it covers one's garment with violence" (Malachi 2:16). Still, God permitted it despite His hatred of it. "Moses, because of the hardness of your hearts, permitted you to divorce your wives, but from the beginning it was not so" (Matthew 19:8).
From the beginning of time, marriage has been described as a union between a man and a woman. "Therefore a man shall leave his father and mother and be joined to his wife, and they shall become one flesh" (Genesis 2:24). The use of singular terms for both the husband and the wife implies that marriage was intended for one man and one woman. Our Lord confirms this: "Have you not read that He who made them at the beginning 'made them male and female,' and said, 'For this reason a man shall leave his father and mother and be joined to his wife, and the two shall become one flesh'? So then, they are no longer two but one flesh. Therefore what God has joined together, let not man separate" (Matthew 19:4-6). Notice that Jesus not only uses the singular for the husband and wife, but he specifically states that the two become one -- two, and not three, four, or five. The implication of one man for one woman is continued in the apostle's writings (Ephesians 5:31). The one-to-one relationship is clearly seen in Paul's instruction: "Nevertheless, because of sexual immorality, let each man have his own wife, and let each woman have her own husband" (I Corinthians 7:2).
The qualifications for elders and deacons found in I Timothy 3 and Titus 1 are not requirements unique to these men alone. Each quality is something that every Christian ought to display. The only difference is that if a man is going to serve as an elder or a deacon, he must possess the quality. For example, an elder is not to be violent (I Timothy 3:3). By your argument, if a society allows violence then a Christian can be violent so long as he doesn't plan to be an elder or deacon -- of course, such an argument is absurd. In formal logic terms, your argument turned an implication into an equivalence; something that formal logic declares leads to a falsehood.
The Bible declares that it contains all that is good for man (II Timothy 3:16-17; II Peter 1:3), but it doesn't necessarily imply that everything bad for man is spelled out. In order to prove your point that Christians are allowed to practice polygamy, you must show a command, example, or necessary inference that polygamy was approved of by God in the New Testament. As I pointed out above, there are several passages that clearly state that marriage is between one man and one woman. If polygamy is godly, upon what passage will you base this conclusion?