You say you want a traditional marriage? Fine, let's go to Wikipedia:
Point being, that even the notion of "traditional marriage" changes over time.
And even if one views "traditional marriage" as being between a man and a woman, it's not like "traditional marriage" (i.e. marriage between a man and a woman) is going away. I mean, what is the moral or legal objections to "non-traditional marriages" co-existing with "traditional ones"?
All said, the same-sex marriage isn't about "the definition of marriage"; it's about equal rights. Law Prof Orin Kerr esplains:
The right to marry has been historically and remains the right to choose a spouse and, with mutual consent, join together and form a household. Race and gender restrictions shaped marriage during eras of race and gender inequality, but such restrictions were never part of the historical core of the institution of marriage. Today, gender is not relevant to the state in determining spouses’ obligations to each other and to their dependents. Relative gender composition aside, same-sex couples are situated identically to opposite-sex couples in terms of their ability to perform the rights and obligations of marriage under California law. Gender no longer forms an essential part of marriage; marriage under law is a union of equals.
Plaintiffs seek to have the state recognize their committed relationships, and plaintiffs’ relationships are consistent with the core of the history, tradition and practice of marriage in the United States. Perry and Stier [plaintiffs in the California case] seek to be spouses;they seek the mutual obligation and honor that attend marriage, Zarrillo and Katami [more plaintiffs] seek recognition from the state that their union is “a coming together for better or for worse, hopefully enduring, and intimate to the degree of being sacred.” Griswold, 381 US at 486. Plaintiffs’ unions encompass the historical purpose and form of marriage. Only the plaintiffs’ genders relative to one another prevent California from giving their relationships due recognition.
Plaintiffs do not seek recognition of a new right. To characterize plaintiffs’ objective as “the right to same-sex marriage” would suggest that plaintiffs seek something different from what opposite-sex couples across the state enjoy —— namely, marriage. Rather, plaintiffs ask California to recognize their relationships for what they are: marriages.
Indeed. California — the state government — was unable to express what legitimate interest it had in the gay marriage ban, i.e., why the plaintiffs' gender relative to one another is important to California. Sure, the state has a legitimate interest in fostering marriage, in general — but banning marriage for certain members of its population does not further that state interest. (In fact, it impedes that interest).