Norms and objectivity

Suppose it's necessarily true that perfectly rational agents always agree on the same moral norms, e.g. "Torture for fun is wrong". Would that make these norms objectively true? If so, than that seems strange. Why would what perfectly rational agents necessarily agree on be the good? Why not what, say, perfectly loving or perfectly selfish agents necessarily agree on? But what if it is also true that 'being rational' is the only property such that agents having that property perfectly, necessarily arrive at the same moral norms. Would that make it more likely that these norms are indeed objectively true?