What Exegetical Evidence do Protestant Evangelicals have to conclude that Song of Solomon is about Christ and the Church?

Indeed, Paul wrote of the "mystery" of a man and a woman symbolizing Christ and the Church. But he also maintained that the marriage relationship was holy. However some aver that the racy, quite suggestive melo-drama / opera of ***the Song of Solomon*** is a picture of Christ and the Church! But what we are told of Solomon's life, he was quite addicted to sensual relationships: a thousand wives! And concubines. How could he be considered an authority--even inspired authority--on connubial relationships? What hermeneutical reasoning guides these theologians?