Let's start with how Augustine, the medieval tradition, and modern Catholicism think about sin. Augustine's understanding of sin is rooted primarily in his Neo-Platonic anthropology. Humans are desiring subjects, Plato and Plotinus tell us. In the dialogue The Phaedrus, Plato tells us that our desires for earthly things (in this case quite specifically, erotic desire- Phaedrus is composing a homosexual love poem!) are simply a misdirected desire for the "Good." Since as most Patristic authors do, Augustine thinks that Plato's "Good" is actually a fuzzy knowledge and affirmation of the Triune God of the Bible, he argues that the essence of sin is desiring things that aren't God. Prior to the Fall, Augustine argues in bk 14 of City of God, Adam and Eve wouldn't have really even have enjoyed sex. Why? Because Augustine says, when you have an orgasm, your mind goes blank. Hence you're not being rational or thinking about God or desiring him. Enjoying sex is actually a punishment for sin, because its a big distraction from doing philosophy/theology and contemplating God.
But does that mean that we shouldn't engage in any earthly activities that we might enjoy? No, says Augustine, but there is a distinction between "enjoyment"and "use." The hard fact is that if don't eat or get married, then the human race will die out- hence we need to "use" these things for those ends, but enjoying them is sin. Hence, Monasticism makes a lot of sense within this scenario. Cut off as many distractions from pure desire for God as possible. In the subsequent tradition, other theologians are a little more tolerant. Aquinas for example compares our movement towards our final enjoyment of God in heaven as being like walking down a road. You can, he says, enjoy the pleasant scenery and you don't have to look at every step that your taking. Nevertheless, everything has to feed into getting to the final vision of God in his essence in heaven and the ultimate enjoyment that brings.
This is the reason why the Catholic Church continues to complain of the ripping asunder of the "unitive" and the "reproductive" aspects of sex. On one level, I agree with the Catholic position that sex can't be divorced from the family, not least because it creates intimacy between a man and a woman who in turn are able to use that intimacy to create a happy and safe home for children. On another level, the Catholic understanding is tied up in an Aristotelian ethic of telos and a Platonic one of our erotic desire for God. The Catholic assumption is that, in a word, sex is a distraction from enjoying God, period. Now, it can be redeemed if in having sex there might be a possibility that you create a human being who will eventually see God in his essence and enjoy him. But simply doing it for fun it is sinful (Pius the XII actually claimed that a person could commit adultery with their own wife!). It is a distraction from enjoy God in a quasi-erotic sense.
This carries over into the bridal mysticism of the Middle Ages, which mainly manifests itself in the forms of allegorical commentaries on Song of Songs. If you think about it, this is actually an incredibly necessary theological strategy in light how the tradition understands sin and righteousness. If Song of Songs is an affirmation of human sexuality for its own sake as a gift of God, then this is a problem if you consider sin to be desiring things that aren't God for their own sake. Hence, one kills two birds with one stone if you make the whole thing about Christ and the Church. Now, this isn't entirely invalid, since Paul does tell us all marriages is an allegory of Christ and the Church (Ephesians 5), and in fact it is a way of reading the poem that goes all the way back to Judaism (one Jewish group I read about actually reads as an allegory of Israel and the coming Messiah, and therefore reads the whole text at a synagogue service every Friday night!). Nevertheless, if you believe that what the text is telling you is that you should actually be having erotic feeling for God, then it gets a little creepy real fast. In fact, much of the mysticism of the Middle Ages that centers on this theme represents a field day for Freudian psychologists.
This bring us to Luther, who as we know, studied Medieval mystical texts from the years 1511-1518/9. In a sense, beyond his use of the Bible and its description of the YHWH/Israel-Christ/Church relationship as bridal, this explains his use of the marriage image for justification in Freedom of a Christian. Nonetheless, as we can observe from the text of this work, Luther uses the bridal image in a very different way. In Luther's reformation breakthrough and prior to this, there is a developing distance between Luther and the Augustinian Neo-Platonic anthropology. The problem of sin ceases to be one of wrong desire and becomes one of wrong trust. Luther comes to define sin as trust in things that aren't God, chiefly, ourselves.
This has two effects on how Luther appropriates the bridal images. First, he doesn't think of Christ relationship to the soul of the believer as one of the union of desiring subjects. We don't have any qualities that would make God feel desire for us (this is the entire Roman Catholic concept of grace- grace makes God find us attractive and thereby desire us). Conversely, we are not the bride of Christ because by grace we have come have quasi-erotic desires for him as the highest object of longing. Rather, Luther uses the image to describe the mutual of exchange of goods between us and Christ (sin and death for life and righteousness). It also describes the self-giving and trustworthiness of Christ. Christ surrenders himself to the believing soul just as the husband surrenders his person to the wife.
The second effect is that desire for earthly things is now ok. In Freedom of a Christian, this expresses itself in the second half with the emphasis on vocation in the created realm. In other words, sin is not loving earthly things, but trusting in an ultimate sense in earthly things. Wanting and desiring earthly things- the stuff of every day life- is ok. In fact, not delighting in them or enjoying them is a sin against God the creator. This rhetoric runs all through the Large Catechism's description of the first article.
On a side note, it's interesting to see how the more things change the more they stay the same. American Evangelicals have definitely picked up on the pre-reformation tradition of sexualizing God in a big way. They constantly write books title things like "desiring God." In fact the whole contemporary worship movement is based on trying to eroticize God. Specifically they do this by playing soft-rock sounding music which reminds people of their romantic encounters down through the years. Playing that music in church gives people a erotic association with church and thereby psychologically manipulates them into attending. Again, also, the whole emphasis is about enjoyment and person potential to become enjoyable to God. Hence, the Joel Osteen "be a better you" books.