As Christians, we are the bride of Christ. If we then in turn start hanging around with some other guy (god) we are fornicating spiritually.
In the Old Testament the book of Hosea presents an interesting story. The Israelites were whores when they turned and started worshipping pagan dieties. But God loved them so much, he took them back. Just like someone may take their mate back who has had an affair and forgives them and continues to love them. That is what Hosea is about.
