Why Are Christians not "allowed to Date Non-Christians?

If a Christian (evangelical?) falls in love with someone who say.....is agnostic or somewhat open to the concept of christianity but doesn't want to commit to it they aren't suppose to date? Aren't you suppose to love everyone despite their differences? If the non-christian is open-minded enough to go to your church, support your choices as a christian(celebicy, prayer, and whatnot) what's the problem? It makes me so sad when religions that promote love, joy, and welcoming to people choose to shun those who don't see eye-2-eye. What's wrong w/marrying someone different from you as long as they're not a jerk? Does that go for catholics as well?