Because the Bible isn’t a one-shot document that God typed up in MS Office in an afternoon. It’s a compilation of numerous texts given to different people groups, through 40+ human authors, over a 4,000 year period. That means each individual book can’t be ready as a simple list of “do this, don’t do that,” but rather must be interpreted in its own literary, historical, and cultural context.
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That is, Song of Solomon is Hebrew Poetry, written at the height of the Davidic Kingdom to a Jewish audience. You can’t read it with the same mindset as 1 Corinthians, written by Paul to 1st century Gentiles as an instructional letter.