@ghug:
It's not just Christianity. Christianity stemmed from Judiasm, and even Islam agrees with rules for marriage.
It's not an issue with the word marriage, but the institution. Think about it like this: Let's look at the definitions of what it means to be a man and a woman. We define men based on physical characteristics, but it goes deeper than that. It goes even to a chromosomal level. We attempt to change the definition of things, but the natural order that has been set forward clearly defines things differently.
Just because we want to call marriage something else, doesn't mean that it changes. A rose by any other name is still a rose. Mankind can call marriage whatever they want, but the truth of it is that one man was designed for one woman in any facet.
When God instituted marriage, it was as a covenant between man and wife to allow for a deeper connection, including a physical union between the two. If we dissolve marriage or allow it to be defined willy-nilly, then what is to prevent people from being joined with animals in a similar union? What about multiple husbands or wives?
An appropriate definition has been established from the beginning of time as to what marriage is, what it was intended for. Humans are selfish creatures. We want things to be OUR way. We cannot understand when something isn't permissible because we fail to understand the purpose and origin of marriage.
Marriage, in it's perfect God-designed format, eliminates sexually transmitted diseases, eliminates broken homes caused by infidelity and divorce, creates a healthy, stable environment to raise children, promotes a positive self-worth among the married couples, establishes a deep bond between two individuals, and does so in a fashion that is life-long. In it's purest form, it is a perfect institution.
We, as humans, destroy it when we are selfish, covetous, envious, adulterous, and self-centered. Because of our sinful limitations, we attempt to do away with the perfect design of marriage so that we can please ourselves. We don't want to have a life-long committment, so we say that we should create civil unions. We don't want to limit ourselves to one spouse, so we say that polygamy should be permitted. We don't like the definition of what marriage is, so we attempt to redefine it to suit ourselves, so that we don't have to feel guilt when we do something that hurts other people.