"For this reason a man will leave his father and mother and be united to his wife, and they will become one flesh." NIV translation
"For this reason" is talking about the fact that the woman was made from the part of the man and so neither are complete without the other. When they find their other half, they can then leave the comfort and protection of their parent's home and face the world as a complete person. It is also a reminder that only these two can come together to be one as God did not remove more than one part from the man. If you have ever seen a broken bone, then, you know that the two halves of that bone are the only two that fit together. You can attempt to put another piece of bone in its place but it will not fit perfectly. When it is the right two pieces of the bone (people) the result is a perfect marriage.
"The man and his wife were both naked, and they felt no shame." NIV translation
Many of us have heard the phrase "ignorance is bliss" and this is an example of the truth contained in the phrase. The man and woman were innocent in that they did not know that evil existed. They knew what good was as they could see it all around them in all that God had created but they were blissfully ignorant of evil.