1. Traditional marriage is defined not just as the union of a man and a woman, but rather as the union of a man and a woman with different roles for the purpose of the formation of a family, i.e., for the purpose of procreation and/or the raising of children. Those who claim that the traditional definition does not include this notion of child-rearing either do not know their tradition or are being disingenuous.
2. The civil notion of marriage contains no requirement or expectation that children will be raised within the union. Though it may have at one time, it no longer does. This is evident from the fact that civil marriage licenses are issued to the naturally and artificially sterile and those beyond child-bearing age and from the fact that not even consummation of the union is required for a marriage to be considered legally valid at this point in time.
3. There is no expectation in the civil notion of marriage, unlike the traditional notion, that the husband and wife have necessarily different roles. Rather, civil marriage is looked at as a contract between two equal partners. Witness the notion of community property and the gradual erosion of notions such as alimony.
4. Moreover, many religious groups which defend fail to consider many civil marriages as true marriages. For example, Roman Catholics do not recognize the marriages of divorcés as true marriages.
5. Thus traditional marriage is not the same thing as civil marriage. In fact, "marriage" is not univocal.
6. In conclusion, the granting of civil marriage licenses to same sex couples cannot harm traditional marriage directly nor change the definition of "traditional marriage" since they are two very different things.