Spiritual Girls are girls that won't say they're Christian when asked about religion for fear that you'll think they're morally strong and that trying to get them drunk and have dirty bathroom sex with them will be fruitless.
Instead they say they're spiritual, which means they'll still sex anyone that can fog a mirror but they don't want you to think they're so stupid and uncultured as to not have any religious moral compass at all.
Sam: What's your religion?
Lisa: I'm a spiritual girl. I like, LIKE the spirit world and crystals, but like, I'm not sure what to think about my soul, you know? Like it's so mystical, and like, stuff....OMG a shiny thing. What was I saying?
Sam (grinning knowingly): This is my friend John. He and his girlfriend Jennifer will be joining us tonight. They're "spiritual" too. Who has the lube?