There is no absolute right, no natural right to marry, not even for heterosexuals. Marriage contracts are issued by the state. It exercises power to control or withhold the right to marry. The reason is that the state has a vested interest in the well-being of its children. The state's overriding interest is that children are the state's future citizens, soldiers, tax payers, farmers, and economic protagonists. In ensuring the future prosperity of the state, the first step is to make sure that there is a father and a mother are legally bound--are forced by the state--to provide for and raise them. That's all there is to it. Marriage is about children. The state has no interest in gay marriage because gays cannot beget children.
The purpose of a government is to ensure the common good. In controlling marriage, the state is acting for the common good. The state's control of marriage is a limitation on individual freedom, but individual rights must be balanced against the rights of the community, the state.
The issue of gay marriage has arisen from what is wrong with contemporary heterosexual marriage. Of marriage, people's heads have become saturated with sentimental ideas about love and romance. to the exclusion of all else. Heterosexuals have been getting married for decades almost solely due to sentimental ideas of love and romance. It is one of the reasons for the high divorce rate today--because romantic feelings of love and romance only last so long, and then what do you do? The primary driver behind the demand for homosexual marriage is that the almost exclusive emphasis on romantic notions of love and romance in the heterosexual community has spilled over and completely infected the homosexual community with the same illusions.
These are all principles. The problem is that the devil is in the details. There are many inefficiencies, exceptions, and issues with these principles.
Having said all of the above, it may shock you, but I am fully in favor of gay civil marriage. The reason is that in America, the way the laws are constructed, hundreds, even thousands of legal entitlements and rights flow from a marriage contract. In the current legal environment, the only way for get people to get them is to have legal gay marriage. If two gay people want to get married, who cares? If two gay people want to enter into a relationship with all of the legal rights of marriage, I think they should be entitled to. They are not harming anyone else.
If marriage is supposed to be about the welfare of children, given the high rate of divorce and the high rate of births from unmarried people, then what we really need is to re-think heterosexual marriage.