The subject of Christians dating people who are not Christians is one to be careful with. It probably doesn't help that the subject only ever seems to be raised when someone is doing it and someone else wants to tell them why it is wrong.
Both sides of the argument about whether it is OK or not refer to a few bits in the Bible, sometimes the same ones! I am going to leave the task of interpreting scripture regarding this to minds smarter than my own. But I would like to describe 4 possible things that a person may consciously or unconsciously mean when they say "It's wrong for a Christian to date a non-Christian", and my thoughts on each motive or reason.