Gen 3:
22And the LORD God said, Behold, the man is become as one of us, to know good and evil: and now, lest he put forth his hand, and take also of the tree of life, and eat, and live for ever:
Deut 1:
39Moreover your little ones, which ye said should be a prey, and your children, which in that day had no knowledge between good and evil, they shall go in thither, and unto them will I give it, and they shall possess it.
The Bible doesn't say that all men are become as one of us, it says the man is become one of us.
I don't agree with every Pelagius stood for, but I think there is a happier median to be found between the two beliefs.
The one great problem of original sin is that it clashes with man's irresistible convictions of justice. These innate, God-given convictions affirm to us irresistibly that it is impossible to hold a man responsible for a deed that he did not commit and that was committed thousands of years before he was born and came into existence. So the theologians who defend the theory of original sin have the impossible task of justifying God for doing what their own conscience affirms he could not be just in doing.