The plumbing speaks for itself. Two males or two females cannot conceive a child together naturally during sex. They can only do it with the help from another female or male. Using the argument of infertile male/female couples isn't accurate because there have been instances where such a couple ended up conceiving despite a doctor's diagnosis. There is zero chance that a gay couple can do it on their own.

That is the essence of the traditional marriage argument. This doesn't mean that gay people are wrong or even incapable of being good parents. It's the natural family unit that is at the heart of marriage and it has been throughout history of the world.