Is a Sin, Always a Sin?

Is a Sin a Sin because it is against Gods Law, in which case we are all sinners and can only be saved by Gods grace alone? Or is what makes a sin a sin based off of the reasons why? Nephi Murdered Laban in order for a generation to not dwindle in unbelief. Soldiers follow orders and even make orders that kill people, if they don’t pull the trigger themselves, Captain Moroni Led his people in the defense of Liberty. Does the reasons why you commit, what others see as, sin have an influence on the nature of that sin? If not should we just bury our weapons of war like the people of Ammon? Or should we stand and fight even if it is the second greatest sin?

Does this principle of the mentality behind sin apply to homosexuality? Is it really a sin to have sex with a Man who is your temporal husband but not celestial? Conversely would it be a sin for me to have sex with a woman who was my temporal wife but not my celestial one? What then is the “sin?” it is obviously not the marriage bed, but the physical act of same gender sex. Would your mentality, you reasons behind the Sin, to raise a family perhaps, negate the “sin?”