Here is a line of thought I’ve tried to develop a bit before:
Too often we Christians are heard as saying something along the following lines: “Your life of casual sex (or cohabitation, or homosexuality) surely must be leading you to feel empty, unfulfilled, and jaded. But we have the solution for those unpleasant feelings!” To which the reply is often: “I’m sorry to disappoint, but I don’t feel excessively guilty or ashamed or unfulfilled. On the contrary, my gay partnership has given me more emotional peace than I’ve ever had.”
In other words, we Christians are often found making Stendahl’s mistake: in our rush to defend our understanding of sin and human flourishing, we too easily assume that the same emotions must be the universal human result of certain behavioral choices. When those expected emotions aren’t present—when Paul, for instance, feels no guilt after persecuting the early Christians—we’re suddenly left wondering what went wrong with our doctrine of sin.
I’m not sure I’ve said it here any better than I’ve attempted to say it in the past, but perhaps bringing in the counterpoint between Stendahl and Bonhoeffer will turn out to be illuminating for others, as it was for me.
Spiritual Friendship 
Where there is no law there is no transgression, so it only makes sense that unless someone already feels convicted internally and is acting against that conviction then guilt would be the effect. Without conviction of the heart, which can only come from “law,” there can’t be guilt, which, I think is why Paul’s guilt is a different sort of guilt than that felt by either Augustine or Luther. Paul was acting as any devout Pharisee would have done and doing what was both just and morally right in his eyes until the road to Damascus. Whatever guilt he felt (and I think I disagree with you here regarding his feeling “no guilt” after the fact) it was the guilt of one who has been surprised by conviction–he wasn’t expecting it. The contrast would be with both Augustine and Luther, neither of whom would have called themselves blameless or righteous before conversion. Augustine was well aware of his guilt before his conversion experience, hence his dabbling with Manichaeism, which had a stark dichotomy of pure and impure, flesh versus spirit. Luther was a theological love child of the Middle Ages, which, having studied this time period in depth for the last decade, I feel fairly reasonable in calling it the “age guilt in search of grace.” When faced with their own guilt, the practical Paul was nonplussed but then got about business; the introspective Augustine was acutely aware and felt the need to expiate for the remainder of his life; and dear old Luther was doomed from the start in a culture that was made paranoid by guilt. So, yes, different sorts of guilt are what we’ve got going on here.
However, as to the actual point that you were making, it seems that Stendahl’s argument is anachronistic for us now. Thinking of the Great Awakenings of the past few centuries and knowing what I do of sermons from the Middle Ages, there was a long time period in which guilt could lead to repentance and to Christ. The threat of hellfire was rather overused, but at the time it worked and was remarkably effective. However, it seems from my own limited experience in the present that what our culture suffers from is not a sense of guilt but a lack of it. As I mentioned previously, guilt must be emotionally internalized in order for it to be wielded as a rhetorical weapon, and guilt can only be felt in two ways: internally as one’s conscience convicts or externally as a law outside of oneself convicts, but either way *something* must convict. The difficulty with many moral issues, especially sexual ethics of all stripes, is that they seem so personal that people have convinced themselves that “the personal is not the political” (to reverse the Feminist adage). In other words, what is done in the bedroom stays in the bedroom and has no lasting effect outside of it. Of course, as Christians we take on faith (but not always, as sometimes we can see it ourselves if we follow the logic train closely enough) that God’s way is the only way to ensure that our lives and world function as they should. To impose that worldview on a secular, pluralistic society is then asking for resistance. In the case of our current culture there is neither an external (God’s will revealed in Scripture, public statutes, cultural censure, etc.) nor an internal (the conviction of the Holy Spirit or conscience) check, and we’re left puffing smoke to the wind because in secular eyes we have no ground upon which to make our argument. Long version cut short, the only people who can feel any call to repentance through a sense of guilt are those who already have an inkling that they are transgressing. For the others, as Christians we’d exercise better tactics to stress the attractiveness of the Gospel and Grace, which seems to be what most churches (that I’m aware of at least) are doing these days. The difficulty, and one that I have yet to find a way around, is in regard to the legal aspects of a moral institution like marriage in a secular society that no longer shares our worldview. How can we maintain, in a spirit of humility and love, the orthodox Christian stance while those on the other side of the ethical fence find any position but their own to be morally intolerable and intolerant? This is an odd conundrum and one that gives me an overwhelming headache.
And yes, God bless Bonhoeffer.