Why is Song of Solomon in our Bibles?

Question: Why is Song of Solomon in our Bibles?

Song of Solomon is in our Bibles to define a Godly marriage. Though Solomon wasn’t perfect later on, here are the details we are given.
Solomon and the Shulamite had a personal relationship with each other, sex was an aspect of their relationship that God approved of, they were committed to each other for life, and did so exclusively to one another in this relationship.
We later apply this to Jesus and the Holy Spirit’s reference through Paul in Ephesians for whenever they mentioned a Godly marriage.