Rona,
The reason I pointed out that sociologists study marriage was not so much that they studied marriage in past societies, but that they can, and do, study marriage in current societies. And, although various aspects of marriage do change over time, or from society to society, studies of both past and current society show that marriage has always had one unchanging basic purpose: to promote unions between members of the opposite sex for the purpose of forming stable family units in which to raise children.
I have already explained that childless couples are accepted in our society. In times past, there was, indeed, a social stigma attached to being childless. But such is no longer the case and, expecially with the advent of birth control, it is considered a personal choice. That does not change the fact that the institution of marriage exists in our society to promote the behavior of entering into a permanent union with a member of the opposite sex with the purpose of forming a family unit in which to raise children.
I think it is important to note that an institution encourages the members of a society to engage in behaviors beneficial to society as a whole, but it does not force them to do so. Each individual is free to decide whether to get married or not, whether to have children or not. Yet, the society as a whole would quickly die out if everyone decided not to have children. So, for the purpose of its own perpetuation, the society promotes the general idea of growing up and having children.
Society sees other benefits in promoting marriage, such as encouraging members of society to mature, to take on responsibilities, to sacrifice, to participate in the raising and training of the next generation, etc. A definition of "find someone to love" does not *necessarily* encourage the same maturation process. (Not to say all unmarried individuals are immature, selfish jerks and all married people are wise, responsible and mature! I am merely addressing the idea that the current definition of marriage promotes a certain process of maturation.)
Finally, the reason I distinguish between "accepting" a behavior and "promoting" said behavior comes from the force behind a social institution. The institution is, as I said, a mechanism used by society to influence the behavior of the members of the society. In comparison, our society accepts single parents, but we do not have a specific social institution which promotes this behavior as being beneficial to society.
If we change the definition of marrige, same-sex unions will be institutionalize. We will have a social institution which promotes this type of union. The problem is that this type of union is not, in and of itself, of any particular benefit to society. So, marriage itself will no longer be of any particular benefit to society.
Edited: Just to clarify, Wendy argues that the very process of allowing the union is beneficial in that she claims it creates a welcoming, inclusive society. But I am not talking about the supposed benefits associated with process of allowing the union, rather the fact that the union itself (as wonderful as it may be for the individuals) does not result in any particular benefit to society as a whole.
