Changing the definition of the word "Marriage", is pursued by activists and Politicians who want to win power, not do what's best for the people. Neither side's goal is an equitable solution.
The issue is useful for scaring people into contributing funds, attracting and/or retaining voters and it keeps government benefit costs down. Politically aligned gay activists want to keep their rabble rousing jobs going indefinitely. Politically speaking, this issue is extremely useful to both parties. Politicians have no motivation to change the status quo. Stirring up controversy is good for their business.
These questionable motivations are antithetical to the interests of same sex couples waiting for legal stature.
The primary motivation of most same sex individuals, who wish to live as a couple (co-join their finances and live together), is to achieve legal rights to estate, medical visitation, government benefits etc., on par with heterosexual couples, which would be recognized by every state.
The issue is not that two people who love each other can't get married and live together. Anyone can get "married" without government approval. The issue is giving legal status to individuals in a same sex union. It is about the Marriage Contract, not marriage itself.
A government issued "marriage" certificate is a secular document to track unions for legal and practical purposes. Using the terminology 'marriage' is misleading and can even be viewed as the government imposing a religion or interfering with the right to association of same sex couples.
People are mistaken in framing the issue as being about the morality of a same sex union.
A domestic/civil union is not about sex! It is a legal document outlining family, financial and access arrangements. It is neither a moral judgment nor justification for homosexuality. Giving legal status to same sex domestic/civil unions, therefore, does not promote or legitimize homosexuality. Refusing to give legal status to partners in a domestic union will not diminish homosexuality either. In fact, due to the publicity of the refusal to give legal status, it increases awareness of Homosexuality, which is exactly what the Gay activists want.
The most equitable and practical solution
Remove the word "Marriage" from all civil documents. All couples, Heterosexual or Homosexual, would be called "Civil Unions", with equal legal rights. Restrictively define a Domestic or Civil Union as a "union of two individual human beings, not already in another union and not directly related", so that it cannot become polygamy, bestiality, incest or a work around for inheritance tax.
Use of the word "Marriage" would be solely in the domain of religious authorities. Only the clergy would be able to perform marriages. Both the clergy and civil authorities can issue a Civil Union license. If a gay church wants to perform a marriage or ceremony the government has no right to interfere. This separates marriage as a private matter, where it belongs.
The word "Spouse" comes from the root to pledge or solemnly promise. If acceptable, the word "Spouse" can apply also to those pledged in a civil union. If that is not acceptable, pass temporary legislation to equate the word “spouse” and “Civil Union partner” for legal purposes (such as insurance coverage, social security, military benefits, pensions, medical leave, inheritance, divorce, hospital visitation etc.) until the wording can be changed in all civil documents.
Issuing both civil union and marriage certificates would create two classes of citizens and foster conflict. Everyone can be treated equally by 'Civil Union" wording. Changing, expanding or overturning the definition of marriage causes conflict. Using the wording "Civil Union" bypasses conflict. In the USA. "Marriage" by legal definition and according to common law, precedent, tradition, Religious belief, Federal and State DOMA (defense of marriage acts) and State Constitutional amendments is considered as solely a union between one man and one woman.
By this legal definition, same sex “marriage” is contradiction in terms. It makes as little sense as allowing for a Male “Lesbian”. By the same token, of legal entrenchment, overturning the definition of marriage entirely, to also apply to same sex couples, is quite impractical compared to the mere changing of the wording of a license. Privatizing by removing the marriage contract completely from government and relying on individual contracts is problematic as long as there are government benefits involved, and recognition by all state governments.
Getting around Politics
Change only comes, (until we fix this broken system where parties rule), when the rulers interests are served. They want money, power and votes. So which party will make this happen? Perhaps, counter intuitively, this is a winning opportunity for the Republican party. If they initiate this legislation, it would sway the many, well to do, fiscally conservative, previously alienated gay and gay supporting voters back to the Republican party and it would not alienate the Religious Right, because the definition of marriage remains the same.
The Democrats have every incentive not to bring up this legislation and risk losing a lock on Gay voters. If the Democrats are forced to bring up a bill, the Democrats will craft wording that is in some way abhorrent to Republicans. In that manner the bill will be killed and blamed on the Republicans. Same sex couples will still be out in the cold.