All we need to do is look in the Old Testament to see why God says not to marry unbelievers. "When the Lord your God shall bring you into the land where you are entering to possess it, and shall clear away many nations before you, the Hittites and the Girgashites and the Amorites and the Canaanites and the Perizzites and the Hivites and the Jebusites, seven nations greater and stronger than you, The reason not to marry an unbeliever is that he/she can influence you away from the Lord.
They have been freed from slavery and are now free men, about to enter the Promised Land.
This may sound harsh, but something as serious as marrying an unbeliever needs to be dealt with properly and to-the-point.
We are not to compromise the will of God and endanger our spiritual well-being.
Additionally, all throughout the Bible God warns us for our own protection not to partner (be unequally yoked) with unbelievers (ex.:2Cor ).
History is littered with the ruined lives of those who have failed to obey this command.
If you are one of the many Christian singles dating unbelievers, save yourself additional troubles by getting out of the dating relationship as soon as possible.