Shigeru-Kun #fundie serebiiforums.com
The reason Christians shouldn't marry outside of religion is because you belong to God.
What good is darkness to light? What common ground could good have with evil? Absolutely nothing.
Even if you love him, he's a child of satan, not of God. You on the other hand DO belong to God. By marrying him you would be taking in satan for a father-in-law and also be harming yourself. Your body belongs to God and when you have sex with someone, you become one. When two Christians have sex that's fine, but if a Christian and a non-believer become one?
You're allowing the devil to step into places he shouldn't be in.
Now mind you, you did say "but we're not getting married" so that does throw marriage and sex out the window...but then why date? As Pikawho? said, dating is used to find a suitable mate. If you're not dating, then just call it friendship! In fact, it's quite dangerous to call it anything else. You may say it won't lead to marriage, but how do you know? One's heart is an unpredictable thing, if you totally fall in love with this boy, you may run into a serious problem later on.
So personally? I wouldn't date. I wouldn't even call it dating. You can still be friends with him, but I'd pray over it. Make sure the Lord is with you and don't let yourself get too in over your head. We shouldn't give up on people, but you don't want to take the chance of falling away from God or doing something that displeases Him.