First, if you're Christian, you should read the scriptures. It says that you should not marry an unbeliever. This is presented as being unequally yoked (2 Corinthians 6:14). An unbeliever will always try to pull you away from God. We should be on a quest for holiness. Holiness comes from God (the Holy One).
Second, why are you sleeping with her? Sex is supposed to be for a man and a woman within the constraints of marriage. Sex outside of marriage was a pagan practice that God punished the Moabites, Amorites, and other pagan nations in the Old Testament. From a practical aspect, sleeping with someone makes it harder for you to make decisions that might break up the relationship. If she wants to leave you for your faith, then sleeping with her would make it harder for you to choose your walk with God over her.
Finally, why does she hate religion? How are you showing her Christianity? This goes back to you sleeping with her, but also how you treat her and lead your life. Do you look like the rest of the world? Or do you look different? Do you carry yourself in a more Godly manner? If you live according to Christ's word, what's to stop you from winning her for the Kingdom of God?