So, in a state where gay marriage was "constitutionally" banned, why couldn't a locality refuse to enforce the unconstitutional ban on gay-marriage?
First of all long before there was any ban on marriage benefits for homosexuals, there were simply no benefits offered to such relationships. Legal benefits were available only to heterosexual couples. So this isn't nearly so much a question of a "ban" as it is what is the definition of marriage.
Is marriage any two consenting adults? What about 3 to 10 consenting adults?
Does marriage presume an intimate, sexual relationship? Or can it be entirely platonic? Ie can two roommates who share expenses honorable and honestly get "married" to enjoy tax benefits and employer provided health benefits? Can an aged parent "marry" a child so as to leave property (and perhaps extend payments on a pension or on social security survivor benefits) while avoiding inheritance tax?
Next, "nullification" is an interesting topic. It is well and good to want local officials to refuse to enforce laws we don't agree with, or that we think violate basic rights. Most folks get less happy about local officials refusing to enforce laws they do agree with. I recall a governor calling out the national guard and himself standing personally in the doorway of a school so as to prevent racial integration. He believed firmly--wrongly, but firmly--in a "right" to maintain racially segregated schools.
Jury nullification extends no further than preventing government from imposing criminal penalties on someone.
State nullification of over-reaching federal laws can protect citizens from federal over-reach the same way federal action can protect from State violations. Generally, proper State nullification would require the actions of the legislature as opposed just one man deciding to act.
In brief, nullification is easy so long as it goes the way we think appropriate. When it goes contrary, we tend to use words other than "nullification" to describe it.
But, government has no business giving tax breaks or any other advantages to married heteros either.
This is a very minority position. It is generally recognized that society/government does have some proper power to encourage/reward desirable conduct. Whether that is tax breaks for buying a home, but not for buying a car or vacation, tax breaks for installing solar, or for investing in retirement accounts, or even making charitable contributions, government routinely provides encouragement for certain conduct deemed beneficial to society.
To remove or contest this ability leaves government with only two options: to ignore conduct, or to punish it criminally. That is a blunt force approach to maintaining an orderly society. The power to encourage conduct, without mandating it or outright punishing non-conduct is an important tool that can be far more nuanced that just the criminal code.
It is well recognized now that children raised by the married parents in an in-tact family are much less likely to engage in criminal conduct or wind up in poverty. Even I, the interventionist statist don't care to see fornication criminalized. Rather, some benefits to encourage men and women to get and stay married seems a far more appropriate course for society to take. Social stigma for those who don't is one part. Official recognition and various legal and tax benefits are another.
However, with all that said, rights are rights are rights are rights. If two consenting adults want to make certain promises to one another and live in a state of marriage, I consider it their right to do so.
What about 3 consenting adults? Or those closely related?
I believe adults have the right to live in peace. I do not believe they have any right to force society/government to grant them the same benefits reserved for marriage between one man and one woman, if their chosen relationship is different than that.
Charles