Don't have one.

Every (every, every) argument against gay marriage comes down to this: Gay people make me sick.

No matter how fancy the argument. How "reasoned" the response. Etc., etc.

It's all a version of "Gay people make me sick."

Just like every (every, every) argument against Brown v. Board came down to: Black people make me sick.

There is no str8 defense of het-only marriage that can possibly be advanced in a democracy founded on radical equality.

None.

If you don't like this, there are alternatives, e.g., monarchy, theocracy, etc.

But stop pretending you're a democrat.