"No, it is not fair!"
Translation:No, non è giusto!
Well, it depends on context; if the question was "Accetti questa decisione?" (do you accept this decision?) your translation would indeed be correct. But the masculine is generally used when the subject is "environmental"; "è buio" (it's dark), "è vero" (it's true), "giusto!" (right!). In this case the adjectives don't refer to any specific word, they're used as if they were adverbs.