It goes without saying that a word can have multiple definitions, and that the definition of a term can change based on common usage. There are currently two definitions of marriage in common use, and the recent Supreme Court decision regarding the Defense of Marriage Act seems to be based on one definition rather than the other.
The definition of marriage behind the Supreme Court decision on DOMA reads something like this: marriage is the public affirmation of an exclusive and loving commitment between two individuals. This is how our culture has come to treat the institution, and it is this common social understanding of marriage that has motivated recent attempts to change the legal institution. Indeed, under this definition, there is no compelling reason to limit the institution to opposite-sex couples. If we say that marriage is about recognizing the value of human relationality, then we cannot discriminate based on sex. As Justice Kennedy’s opinion suggests, asserting that genuinely human and fulfilling relationships can only occur between one man and one woman wrongly demeans every other kind of fulfilling interpersonal relationship.
Marriage, under a more traditional usage, is a loving and exclusive sexual relationship between a man and a woman that is both open to the creation of new life and ready to take responsibility for that new life. This definition of marriage involves the public acknowledgement of an exclusive and loving relationship between individuals, but its limitation to opposite-sex couples is justified in that marriage is understood as a way of harnessing both sexuality and emotional attachment as a means for creating families: through marriage, children can not only be created, but brought up with their biological parents as their primary caregivers. Defenders of traditional marriage are not arguing that loving relationships between adults have no value, but rather, that the difference between a generic publicly-affirmed loving relationship and one that exists between a man and the woman for the purpose of creating a family is significant enough for the latter idea to have its own exclusive term.