Veteran's Administration hospitals and programs exist because it makes sense to take care of the health and well-being of the men and women who put their lives on the line to serve and protect our nation.
Farmers receive special government-subsidized loans based on the idea they'll help to ensure that the nation's food supply is stable.
Professional sports stadiums and arenas are often subsidized by taxpayer funds, with the expectation that the local economy will benefit from dollars spent by sports teams and fans.
Whether you agree or not with these government programs and practices, it isn't hard to understand the theory behind them, that the government extends benefits to either create new benefits (i.e. student loans), or to reward benefit already received (i.e. VA programs).
On the flip side, I think you'd agree that it doesn't make sense for the government to extend special benefits to programs or practices that provide no return on investment, that yield no benefit to society.
So why would the government encourage, promote and reward gay marriage?
A foundational stabilizing and perpetuating pillar of society is marriage between a man and a woman, who join together in solemn committed matrimony to conceive, bear and raise children in a stable home environment.
No, marriage between a man and a woman is not a Constitutional or civil right. It is for societal protection and preservation. As traditional marriages and families go, so goes society. That is why it is officially recognized by the government, and is encouraged and rewarded via tax breaks and other benefits.
Gay marriage, on the other hand, provides no benefit to society, and no benefit to society should mean no benefits or encouragement from government.