Let us consider a simple mind experiment. Lets imagine Earth was completely destroyed by an asteroid and that mankind have never come into existence. Hence there have never been taxes, rapes, murderers, neither donations, love or adoptions. None of these things ever existed. Would in a scenario like this, tax be immoral? If morality was an objective reality, it would be independent of our existence.
Please, consider that a subjective morality does not imply that each person will have his own and that there is no such a thing as right or wrong. To claim that morality is subjective implies simply that morality is a part of our 'autistic' world, as isacvale said, of the human world, not of the rock and space world of the physics.