JOEL - I think you are right when you say that we don't have the capacity to understand it; at least not right now. But even though plural marriage is an eternal principle, it is not something that is required of everyone in order to obtain eternal life in the Celestial Kingdom. We are only required to accept the truthfulness and necessity of the doctrine in certain circumstances.
One thing that might help in understanding the existance of it is to consider the fact that, unfortunately, the number of righteous women probably outnumbers the number of righteous men, even in the celestial kingdom. If you think about it, what are the odds that there will be exactly the same number of righteous men as there are righteous women who can be sealed to each other and not have any left over? At the present time there are about 700,000 more women in the church than men.

President Brigham Young said:
"If we could make every man upon the earth get him a wife, live righteously and serve God, we would not be under the necessity, perhaps, of taking more than one wife. But they will not do this; the people of God, therefore, have been commanded to take more wives." (Journal of Discourses, Vol.16, p.166 - p.167)

From a mortal perspective, this means that when a man is limited to only one wife, some women will have the choice of marrying a worldly, carnal man or remaining unwed. If men were eternally limited to only one wife each, some women would never have the opportunity for exaltation.
God knew this would be the case, so He uses plural marriage to remedy this problem by enabling every woman the opportunity to have a righteous husband and enjoy the blessings of eternal life.
It may be hard for us to understand it now because our minds and perspectives are limited to this mortal body we are living in now. During the millenium and after, when we are resurrected and become immortal, we will probably be able to understand and accept it better.

Those destined for the Celestial kingdom will come forth in the morning of the first resurrection. During the Millenium and in the Celestial Kingdom, they will have perfect love and acceptance for everyone else and for every doctrine of God. There will be no selfishness, no jealousy, no sin, no reason nor desire for anyone to reject any of God's commandments or principles, including perhaps plural marriage for those who need it.
Imagine you had a sister who you loved very much, who never had the opportunity to marry in this life. And during the millenium there weren't enough men for every woman to marry. The only chance she has for the Celestial Kingdom is to be sealed to someone who already has a wife. Chances are you would not disagree to having her sealed to your righteous husband.