I appreciate your interest to my contribution! You have made an excellent distinction between the interpretations of people with different ecclesiological (Puritans vs Catholics) and philosophical (neo-anthropologist) backgrounds.
There is no denying that at its most basic level the Song of Songs discloses a very intimate and detailed sexual relationship. This first point is of great importance because it is the lens by which all other potential insights from the text are examined - for a Puritan Christian, to whom all sexual matters (for argument's sake) must be a hidden, perhaps even shameful affair, the inclusion of this book is at best an embarrassment and at worst an account of Jewish depravity.
For a (catechised) Catholic on the other hand, this book is conclusive proof against the claim that the Catholic religion hates sex - indeed, an entire book of the Bible happily glorifies in sex, and speaks to the joy of what Catholics believe sex should really be (namely, the fullest body-and-soul expression of eternal love between husband and wife).
It's a lot harder for first-language Anglophones to speak about the Orthodox and other Eastern Christians (since their cultural milieu and theological heritage have developed so differently to ours in the Latin West), so please forgive me for leaving them aside.
In my experience most people (believers and non alike) have no exposure to the more obscure books of the old testament - if it doesn't feature Adam and Eve or Moses, people generally don't know about it. This means that when they do stumble upon it, they are generally unprepared - they don't know what to make of it, what it means, why it's even in the Bible. If one is not familiar with the characterisation of God as the victorious bridegroom delighting in his wife, or Israel as the "land that will be married" (both from the Psalms) how can they be expected to read the Song of Songs as God delighting in his eternal marriage to Israel?
This comes back to my point previously about intrinsic and extrinsic consistency - with a text as obscure and confusing as the Song of Songs, one must be suitably prepared to fit it into the greater framework of Sacred Scripture - familiar enough with the other books of the Bible to fit this one into its proper place, and interpret it in light of the others. If taken by itself, without that greater context, then the Song of Songs absolutely does become nothing more than ancient erotica.
But then, returning to context Christians (and Jews, and in a slightly different way Moslems) believe that each book of scripture is divinely inspired, and has a rightful place in man's religious duty to God. Since erotica for its own sake is a selfish act (one that aims at gratifying oneself alone, rather than ordering all things to the greatest good), to conclude that the Song of Songs is simply ancient, self-gratifying erotica places it at odds with the belief that the entirety of sacred scripture is right and good.
You could probably tell me more about what this means to an agnostic, if you have lasted through my ramble. Apologies for my lack of brevity, I sometimes value comprehensiveness at the expense of straightforwardness.