Example:
The legislature of Utah I think, came within one vote of declaring that legally the value of Pi was henceforth 3.2. It was easier to use that way.
Now. If it had passed, it would have been de jury legal. So what?
Would you want the engineers who design your high rise buildings or your jumbo jets to do the legal thing? or the rational thing?
In other words, you can't pass laws that contradict reality.
The definition of marriage since prehistory is simply reality put into words. Not subject to every perversion's whims.
Your statement presupposes that gay marriage would contradict reality. That's an usupported assertion. It's also quite an ambiguous assertion. If gay marriage is codified into law and embraced by society, how is that contradicting reality? Are gay couples really just an apparition?
The value of Pi is mathematically certain. Societal norms and laws are fluid.