You say you want a traditional marriage? Fine, let's go to Wikipedia:
[Marriage] In the Hebrew Bible (Old Testament)
Rembrandt's depiction of Samson's marriage feast. The Hebrew Bible (Christian Old Testament) describes a number of marriages, including those of Isaac,[Gen 24:49-67] Jacob,[Gen 29:27] and Samson.[Judg 14:7-12] Polygyny, or men having multiple wives at once, is one of the most common marital arrangements represented in the Hebrew Bible,[67] yet scholars doubt that it was commom among average Israelites because of the wealth needed to practice it.[68]
Betrothal (erusin), which is merely a binding promise to get married, is distinct from marriage itself (nissu'in), with the time between these events varying substantially.[67][69] Since a wife was regarded as property in those days, the betrothal (erusin) was effected simply by purchasing her from her father (or guardian)[67][69]; the girl’s consent is not explicitly required by any biblical law.[69] Like the adjacent Arabic culture (in the pre-Islamic period),[70] the act of marriage appears mainly to have consisted of the groom fetching the bride, although among the Israelites (unlike the Arabs) the procession was a festive occasion, accompanied by music, dancing, and lights.[67][69] To celebrate the marriage, week-long feasts were sometimes held.[67][69]
In biblical times, a wife was regarded as chattel, belonging to her husband[67][69]; the descriptions of the bible suggest that she would be expected to perform tasks such as spinning, sewing, weaving, manufacture of clothing, fetching of water, baking of bread, and animal husbandry.[71] However, wives were usually looked after with care, and bigamous men were expected to ensure that they give their first wife food, clothing, and sexual activity.[Ex 21:10]
Since a wife was regarded as property, her husband was originally free to divorce her for any reason, at any time.[69] A divorced couple were permitted to get back together, unless the wife had married someone else after her divorce.[Deut 24:2-4]
Good times.
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).