God's longing for a bride was the secret that He had kept for all eternity. We know from other places in the Scriptures that God is Spirit. But we are also told that God is Love. There was a love in God that was a part of His nature, that He wanted to pour out for all eternity, but there was no place for that love to go, for God was alone. Angles can't understand this love.
Angels are neither male nor female.
They are neuter. But God is love and His eternal purpose involved having someone upon whom He could pour out that love. God's purpose has been kept hidden. What has been on His heart has been cloaked in a mystery. There is also another aspect of God's purpose that should be mentioned.