My argument to gay “marriage” is simple: ask the question “why 2?” The question to ask to gays who want to “marry” is also simple: “You propose the definition of marriage to be a legal contract between 2:n people of either/any gender?”
We can provide a rationale for Marriage between a man and a woman: it is how progeny are created, progeny create families, families with the parents create social stability, social stability is desirable and is thus recognized and supported by governments.
We have thousands of years of background supporting this thesis and structure.
When the (inevitable) response is “but people get divorced (etc.),” the answer is “people may or may not AVAIL THEMSELVES of the structure, but this is the rationale for the structure.”
We are left with the proposed definition I started with (2-n people, any sex). If the other person accepts the definition, we are left with “OK. marriage doesn’t exist at all then.”
It is irrefutable.
...to ask them if they know what a non-sequitur is. I then give them an example: If A equals B, and B equals C, then X equals Y. Another such example is: if 50% of marriages end in divorce, then we should redefine marriage. I always hammer them on this whole '50%' argument. I always press them to explain it, and I never let them slide around it. Then, I use it to set the meme that their logic is flawed.