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 to be holy. (Hebrews 13:4) And the mention of the "Bride" in Revelation 19:7 and 21:2 depicts a more modest picture. However some have avowed 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, and justifies this Christian interpretation: Christ and the Church?*** Note: many Protestant Evangelical denominations pride themselves in their "Historical-grammatical-literal" approach to interpreting the Scriptures. If this book were taken that way, it surely would have to be rated PG-13 or even R-rated ("modesty" is not its main suit). Some theologians admit that it depicts is an erotic poem of sensual love...and even shameless lasciviousness. But for theologians (and mature audiences), it was and still is today by some, considered to be a book that should be taken "allegorically" of Christ and the Church. What justifies this symbolism approach?