It's one of the easiest ways to distinguish God's church from others.

That's not to say that I think this doctrine is changeable. But I also know that I don't know all things. For example, my faith in the reality of modern revelation in general is stronger my faith in many of the individual things that have been revealed thereby.

My faith in the things that are revealed to be true to me by God is stronger than my understanding of how they fit together. So I, personally, don't feel comfortable saying "This is the thing that I wouldn't accept if God told me to". I would have to spiritually evaluate things as they came.

That's not harder for me to do than it would be for Nephi to slay Laban

Not harder than Abraham sacrificing his son

Probably not harder than the Jews accepting Jesus, the fullness of the Gospel and essentially the abandonment (we see it as fulfillment) of the Law that was given to them from God by Moses. That was a complete change in what they understood to be God's law and doctrine.

So I refuse to say, right now, that I wouldn't follow God's prophet in a thing.

But I think the eternal nature of heterosexual marriage is pretty clearly eternal.