They would be "in error"?

True, "heretic" is a very specific charge.

There's really no reason for the dramatic title, is there? The fact would be: they are in error regarding a serious matter of morality. That's nothing to take lightly.

Morality is what determines right and wrong. Without objectively correct morals, we can't make it to heaven.

The laws of God are objective; they apply to everyone.

Even if someone is ignorant of one or more of God's laws, there will be consequences (both natural and supernatural) for breaking them.

If you fail to read the manual and end up damaging your car, is your car any less damaged because you did it out of ignorance?