I think the best way to address this is by means of analogy. Suppose your grandfather inherited great wealth that he himself had not earned. Unfortunately, he also had an unhealthy interest in gambling. Before he died, he went to Las Vegas and lost every penny. You, as a grandson, would have been rich, but instead, your inheritance is reduced. This is the case with original sin. God gave Adam a gift he did not earn, Adam lost it, and so his descendants did not inherit it. But God is still fair to us because he offers us not just the exalted state in which Adam was created, but a much higher state of union with him through Christ. What we have to do is accept the offer.