I'll make a few brief observations about the Song of Solomon:
i) It seems to be attributed to Solomon, and the setting reflects the Solomonic court. Likewise, Solomon is repeatedly mentioned. And the plot only has two main characters.
ii) An objection to Solomonic authorship is that given his notorious promiscuity, he's a poor candidate to celebrate devoted love.
One counterargument is that this work celebrates the love of his life. His one true love. Or first love. Moreover, it may have been written early in life, before he become so degenerate.
There are, however, two other problems with the objection:
a) A minor correction is that his promiscuous reputation is somewhat exaggerated. The fact that he had 700 queens and 300 concubines doesn't mean he was sexually active with all–or even most–of them. Many were political marriages to seal a military or economic alliance with a neighboring city-state or nation. Likewise, harems were inherited. But by them, some of the harem girls were getting up in years.
This is not to deny that Solomon was promiscuous–even exceptionally promiscuous. Unfortunately, it's not uncommon for kings to exploit their position in that respect. Classic abuse of power.
b) In addition, there's no reason to presume that this work is strictly or literally autobiographical. Love stories and love poems are often fictional. They celebrate a romantic ideal. Indeed, poets sometimes resort to fiction in this regard precisely because they yearn for something that real life disappoints. It's a fantasy of how they wish things were. And the advantage of fiction is that you can control the action. The disadvantage is that fiction lacks the vividness of genuine experience.
So it's possible that while the work is informed by Solomon's sexual experience, the work itself is fictional. If so, then it could well celebrate monogamous love. People can sincerely honor and desire something they personally fall short of attaining. Indeed, their very failure may heighten the contrast between the hoped-for ideal and the frustrating reality.
Even if it's "based on a true story," the poet may exercise great literary license. It's hard to say how much is imaginative. And I don't think that detracts from the value of the work. It's not a historical narrative like Genesis.
iii) Another objection is that this work doesn't specify marital love.
However, one problem with that objection is that the lyric poetic genre leaves the narrative sequence somewhat ambiguous. The plot may not be rigorously linear. Rather, it may circle around in flashbacks or erotic dreams.
Scholars differ on whether or not there's chronological progression. Is it single to married? Sexual coming of age (e.g. Garrett)? Or is the couple married throughout (e.g. Hess)?
But the gist of the narrative seems to be courtship followed by marriage. A couple falls in love. They experience intense physical longing to consummate their love. They are tempted to prematurely gratify their passion. So the first part accentuates breathless anticipation.
In the second part we have the honeymoon. This is initially rapturous, yet the bride experiences a sense of letdown. After the initial climax, the rest of the poem describes the development of a more stable, enduring love. It gives the reader the typical ups and downs of falling in love, the wedding night, and the adjustment to life together. Anticlimax follows climax. What do you do for an encore?
Also, insofar as the predominate narrative outlook of the work is from the perspective of the heroine, the bride is more likely to feel a sense of letdown than the groom.
That's perfectly consistent with a monogamous storyline. They keep their passions in check before the wedding. That's somewhat obscured by the impressionistic quality of the presentation.
iv) Thus far I've been viewing the work as a tribute to romantic love. And as one commentator notes, the canon would be unbalanced if it only addressed sexuality in terms of prohibitions, rather than a positive treatment.
v) There is, however, a long history of allegorical interpretations. One of the most ironic is the Marian interpretation, favored by some Catholic commentators. That evinces the desperation of Catholics to prooftext their Marian dogmas. In addition, this work furnishes singularly intractable material to illustrate the perpetual virginity of Mary!
vi) It's sometimes said, even by otherwise reputable scholars (e.g. Clines), that the account was allegorized to make it into the canon–or keep it in the canon. But other scholars says there's precious little evidence that it's erotic content was an obstacle to canonization.
If Scripture can be very explicit about sexual immorality, why can't it be fairly explicit about marital love?
Moreover, the imagery isn't really explicit. Rather, the poet skillfully uses provocative comparisons. The detailed descriptions are not about erogenous anatomy, but figurative analogues (e.g. a cluster of grapes). So there's nothing "pornographic" about the imagery. It uses prosaic words for sexual anatomy. The elaboration shifts to figurative imagery.
vii) Certainly the Bible uses marriage as a theological metaphor. That's deployed by turns to illustrate fidelity and infidelity. The marital metaphor is an emotional resonant and powerful image to illustrate the possessive and protective nature of God's love for Israel and/or the Church.
But any analogy includes disanalogies, and the allegorical interpretation of the work would accentuate the disanalogies to an incongruous degree. Lovesick men and women have limited control over their feelings. The beloved can elicit emotions from the lover. It's an involuntary response.
Surely that's a very unsuitable illustration of how God and Christians interrelate. Do we tug his heartstrings the way the beloved in this poem affects her lover? Is God crazy in love with sinners? Must he keep his passions under tight rein?
I think the poem is clearly describing human romance. That can become a basis for theological metaphors, not vice versa. Not in this instance.