The doctrine of Original Sin has many opponents, including many Christians, but this is largely because it is misunderstood. There are two principal definitions of original sin: the act of sinning committed by Adam and Eve, and the state that humanity is now in due to this sin. To add to Quizro's writeup:

OBJECTION FOUR: "How is Original Sin transmitted? How can a newborn, innocent of all action, 'get' original sin? Is it genetic or something?"

A RESPONSE: Original sin is not magical; it is not a literal stain; it is not a substance; it is not a virus; it is not genetic. When all humans are said to have original sin, it merely means that we are in a negative state--that we lack special graces that we don't truly deserve but that God initially gifted Adam: immortality, the guaranteed vision of God, freedom from disease, complete control over temptation while also having free will, etc.

So the rationalist's objection is thus invalid. The objection from those who quote Ezekiel 18:20 ("It is the soul that sins, and no other, that shall die; a son shall not share a father's guilt, nor a father his son's. The righteous man shall reap the fruit of his own righteousness, and the wicked man the fruit of his own wickedness") is also invalid, because original sin is not a punishment. It is merely a retraction of certain gifts because we don't deserve them. It is also important for Christians to always keep in mind that Original Sin, while real, is fairly unimportant compared to the great gift given to us that is much greater than what was taken away earlier: the love of Christ, and his death for us.