The doctrine of original sin, as I understand it, is that everyone is in some way born guilty, not guilty of having done anything wrong personally (that comes later), but guilty because of the sin of our first parents.
This is a confusing and unpleasant doctrine, and I'm not sure I can make it pleasant, but I can point out that this doctrine, which is so easy to reject in the Christian faith, is something we accept almost without comment or complaint in everyday life.
For example, suppose a company has committed fraud and the guilty management team goes to jail or is fired. And suppose a new, honest, team is installed. Does the new team's innocence mean the company does not have to repay those it defrauded? Hardly. Does it mean the government is unjust if it penalizes the company? Of course not! Though nobody currently at the firm is guilty of anything, the company itself is guilty and everyone who works for it takes some of the burden of that guilt upon themselves, not as personal guilt, but as corporate guilt.
So it is in this sense, I suspect, that the human race is guilty. Though we did not commit Adam's sin and are not individually guilty of anything he did, yet in our identifation with Adam, we are harmed by his sin. And it is no more unjust that this be so than it is unjust for a company to be penalized for the sins of departed directors.