Fill in the blank: marriage is between a man and a ________ ?

Human culture is part of nature, what do you even think nature means? Natural behavior is nature, non-Christian civilizations often paired the same way, even some animals mate for life.
They're probably referring to the fact that "marriage" is a thing/idea/relationship/construct created by humans, and so such idea is subject to changes that are dependent on the will of those who formulated it/follow it (in this case society), and because of it, it's very nature is malleable.

So for example, if you're a father, that relationship is set in stone by a natural process, that's what we call the result of this cause-effect.

Some person/group of people saying marriage is only between man and woman is not immutable, over the years this vision has changed and is now accepted as the union of two people in such a way, regardless of sex.

Whatever it says on an old book is also the word of humans, so unless you go full religious on me, I think what I've said is sensible. Also, if you like semantics, we can just say that another form of marriage exists, as language also evolves, even if it's not considered "true" by the Vatican or whoever you prefer.
 
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