One of the issues around sex work, even in countries where it is legal is how the stigma associated with sex work leads to increased negative outcomes for sex workers.
I think they have an internal feeling of right versus wrong, and then they seek out belief structures that justify their feelings.
That means all this psychological discussion of "feeling dirty" and "justifying their feelings" is attacking a straw man fallacy.
I think that is a weak reframing of the breadth of sex, and that sex for the supermajority of American people, in and out of the church, is pre-marital.
I think young Christian boys are taught that sexual fantasy and pornography is the same as adultery, a defilement of the temple that is their body. I think one example that's brought up along these lines of narrative is a story of direct divine punishment for the spilling of seed. I think Protestant and non-denominationals with sizeable youth groups form little sexual discussion groups, especially for boys, where they vent their sexual guilt with regards to their fantasy of the day or how they masturbated. I think American Christian parents are ever anxious about government discussion of sex, and that they backwardly expend finite energy to make it a political battle point. And I think American Christian culture is as much about the implicit as it is about the explicit.
But somehow your choice framing of the discussion is that Christians don't feel dirty about sex because they are explicitly taught that only sex outside of marriage is dirty.
Young boys and girls in the church learn the anxiety of the community and their parents first, and then they listen to the belief structures to justify that anxiety.
A straw man, by the way, is a misrepresentation of someone else's discussion, and not actually a logical fallacy at all. It is by definition outside of logical purview. Most people would be better off not ever hearing about logical fallacies, because it gets in the way of quality thinking.
Anyways, I want to make one last point about American Christians expending their precious, finite energy in politics. I say that there are 3 high consensus Christian political issues in America, in order of strength: (1) abortion, (2) homosexuality, and (3) sex education in school. The latter is the weakest, and mostly in state-level politics, but there it is brutally fought. Notice the theme of these 3 issues?
It's really kind of silly that these are the highest consensus issues for American Christians. Um, not poverty, right? Or injustice in the police system, and classism and racism in America? Yes. Let's discuss abortion, homosexuality, and sex education in schools (condoms detract from abstinence!).
I made no reframing of the breadth of human sexuality, because that was not what was being discussed. I did make assertions regarding the Christian teaching of what was moral in regard to human sexuality.
You then state a number of assumptions (prefaced with "I think") which are largely irrelevant. The story of Onan, who spilled his seed during intercourse rather than give his brother's widow a child as the Mosaic law required, was punished because of disobedience to the law. That had nothing to do with fantasy or pornography.
Christian teachings regarding sexual fantasy or pornography are all aimed at the training of one's conscience to avoid behavior that typically leads one to the societal norm of extra-marital sexual experience. This expands to a wider belief regarding self-control. If you routinely do something wrong, you come to accept it as normal, and are likely to continue where that path winds.
A straw man argument is a logical fallacy [1]. It is the deliberate misrepresentation of someone's position in order to make it easier to attack. My opinion is that understanding logical fallacies (formal and informal) is important for conducting honest and rational discussions, so we disagree on that.
I tend to agree with your point about wasted effort regarding enforcement of morality with law.
And although moral outrage against sex may not be in the teachings of Jesus, it's certainly in the teachings of Paul and other canonical early Christian writers.
I believe the correct legal thing to do is to criminalize only the buying side of the transaction. If you pay for it, you are victimizing, and that should be criminal. If you provide, that's legal, but not moral.
Decriminilizing it is about improving safety for sex workers. I think that is where our questions of morality should focus. People are going to continue to have sexual needs. Prostitution gets called the oldest profession for a reason: Even certain monkey/ape species will trade sex for goods (like food). Making it illegal doesn't make it go away. It just makes it riskier and that risk disproportionately negatively impacts the women doing sex work.
For that, the Nordic model actually works. Full decriminalization doesn't. http://mic.com/articles/112814/here-s-what-s-happened-in-swe...
Two C.S. Lewis quotes on the subject:
"The sins of the flesh are bad, but they are the least bad of all sins."
"...a cold, self-righteous prig who goes regularly to church may be far nearer to hell than a prostitute."
The desire for sex is ingrained in human nature, and so long as people can pay for things, sex will be one of the things they pay for. If we can reduce the number of victims by making it legal and regulating it, then I'm all for it. But, at the same time, I'd consider myself a failure as a moral guide if my daughter chose it as a career.