I think you enter a difficult zone here, though. If, as you contend, all morality is relative, it's hard to contend then that the morality of today is not as strong and absolute as any morality ever was. If there is no over-arching moral principle, so what? It just means that the only standard by which we can judge is the one we have now. It's a mistake I think many make when dealing with this kind of stuff. Values may not be absolute, but they're still values.
I am not a fan of presentism.