It isn't about God, it doesn't really talk about religion, and it is very obviously erotic in nature. It just seems out of place.
It has generally been interpreted allegorical as the love between God or Christ and the Christian Church, as well as mention of the Virgin Mary ("hortus conclusus" 4:12 - Enclosed Garden).
To add to the Christian interpretations that have been mentioned, the Jewish tradition also sees it as a metaphor for the spiritual connection between God and His people. As the Midrash states:
"[A]ll of eternity in its entirety is not as worthy as the day on which Song of Songs was given to Israel, for all the Writings are holy, but Song of Songs is the Holy of Holies." (cited here on Wikipedia)