Not sure if it's forbidden as much as "for your own safety" type of things. :-) Seriously though, I'm not entirely sure. Maybe because God understands jealousy at its core?

Aside from the obvious reasons, I can't think of a scriptural basis, at least not off the top of my head. Then again, I'm not a Biblical expert or anything. Hmm... I may have to look into the matter tonight. Thanks for an interesting question!

another means of control

Because most of Christianity as it exists today has been strongly influenced by old western romantic notions which, while not scripturally based, have guided much of the thinking of Christianity for so many centuries that they have become identified with each other. - In any case, it is more of a Christian tradition than a Christian doctrine. - To be fair, there does exist a rather narrowly applicable statement by Paul that bishops should be husbands of one wife, but this statement, can by no means provide the foundation for the strong social taboos that exist. - I, for one, think that delving into the realm of having more than one wife can be dangerous, but I note that such practices seemed to be expressly condoned in old testament times. Nowadays we see this practice shunned by the pious leaving the wicked and lecherous almost exclusively to inform our view of what the multiple wife lifestyle is about. This presents a skewed view, as we do not see what a good man in a polyamorous culture might result in. However, the current situation is not without its valuable lessons to society regarding the dangers, for it cannot be denied that they are real. Such multiple-wife arrangements can and have been used in the past and present to enable horrible abuses from rape to incest to pedophilia. - I think society's current standard is probably safe. Most current attempts to break with the standard are not properly motivated.

"Saint Augustine saw a conflict with Old Testament polygamy. He writes in The Good of Marriage (chapter 15) that, although it "was lawful among the ancient fathers: whether it be lawful now also, I would not hastily pronounce. For there is not now necessity of begetting children, as there then was, when, even when wives bear children, it was allowed, in order to a more numerous posterity, to marry other wives in addition, which now is certainly not lawful." He refrained from judging the patriarchs, but did not deduce from their practice the ongoing acceptability of polygamy. In chapter 7, he wrote, "Now indeed in our time, and in keeping with Roman custom, it is no longer allowed to take another wife, so as to have more than one wife living." [emphasis added] However, the Roman Catholic Church has subsequently taught on more fundamental grounds that "polygamy is not in accord with the moral law. [Conjugal] communion is radically contradicted by polygamy; this, in fact, directly negates the plan of God which was revealed from the beginning, because it is contrary to the equal personal dignity of men and women who in matrimony give themselves with a love that is total and therefore unique and exclusive." (Catholic Cathechism, para. 2387, Vatican website). This is also the normal position among Protestant Churches, and it can therefore be said that the mainstream Christian position is to reject polygamy in principle. Periodically, Christian reform movements that have aimed at rebuilding Christian doctrine based on the Bible alone (sola scriptura) have at least temporarily accepted polygamy as a Biblical practice. For example, during the Protestant Reformation, in a document referred to simply as "Der Beichtrat" ( or "The Confessional Advice" ), Martin Luther granted the Landgrave Philip of Hesse, who, for many years, had been living "constantly in a state of adultery and fornication," a dispensation to take a second wife. The double marriage was to be done in secret however, to avoid public scandal. Some fifteen years earlier, in a letter to the Saxon Chancellor Gregor Brück, Luther stated that he could not "forbid a person to marry several wives, for it does not contradict Scripture." "Ego sane fateor, me non posse prohibere, si quis plures velit uxores ducere, nec repugnat sacris literis." "On February 14, 1650, the parliament at Nürnberg decreed that, because so many men were killed during the Thirty Years’ War, the churches for the following ten years could not admit any man under the age of 60 into a monastery. Priests and ministers not bound by any monastery were allowed to marry. Lastly, the decree stated that every man was allowed to marry up to ten women. The men were admonished to behave honorably, provide for their wives properly, and prevent animosity among them." The modern trend towards frequent divorce and remarriage is sometimes referred to by conservative Christians as 'serial polygamy'.[citation needed] In contrast, sociologists and anthropologists refer to this as 'serial monogamy', since it is a series of monogamous (i.e. not polygamous) relationships. In Sub-Saharan Africa, there has often been a tension between the Christian churches' insistence on monogamy and traditional polygamy. In some instances in recent times there have been moves for accommodation; in others churches have resisted such moves strongly. African Independent Churches have sometimes referred to those parts of the Old Testament which describe polygamy in defending the practice." Source and further information: http://en.wikipedia.org/wiki/Polygamy#Christianity

This sounds like a question for the Christian category, not the Mormon category.

Because "no man can serve two masters."

Because God deseigned a relationship for husband and wife. If you have more than one you cant fulfill your Biblical duty for the relationship.

i think that the reason that Christianity as a whole believes it to be wrong is because the first marriage between Adam and Eve was a special relationship that GOD joined them in. but i do understand that even Jacob (whose name would later be changed to Israel the father of the twelve tribes, what GODs people would take as their namesake, the name for their nation) took more then one wife. so there are places where it is allowed and even excepted, but the fact that GOD joined Adam and Eve in this special relationship not Adam, Eve, Sarah, and Rebbecca should give you something to think about.

Because that is how the family arrangement was first designed by God in the Garden of Eden. That is why in the second chapter of Genesis it says "That is why a man will leave his father and his mother and he must stick to his wife and they must become one flesh." One flesh, one of each spouse, one family. That is how it was at the beginning. I would imagine since God is the one who created this couple, he is the one that would show them what a family was.