Lets say the hypothetical situation occurred where the federal government had never passed DOMA and instead had decide to call any marriage contract that 2 consenting adults entered into a domestic partnership. Domestic partnerships would have conveyed all the federal benefits for marriage that we have now without distinguishing between the sexual or religious orientation of the 2 parties. The term marriage would then only be applied to the ceremonial aspect of the union between 2 adults and it would be left to the priests, notaries, etc. to decide if they were comfortable with marrying people who lead lifestyles they disagree with. In this case, gay marriage wouldn't be legal or illegal. Marriage law at the federal level would just be recognized as essentially what it is, which is contract law.
I think much of the problem with gay marriage comes from the use of the term marriage which many people take to have a religious connotation. Calling only gay marriages civil unions is considered discriminatory. Would you, who are opposed to legally sanctioned gay marriage, have an issue with federal benefits being attached to the partnership contract for all couples and having the federal government not say one way or another if gay "marriage" was legal? This is just out of curiosity and not meant to be accusatory or questioning anyones beliefs.