(I see you've been downvoted. It wasn't by me. I very seldom both downvote someone and bother to argue with them.)

I don't care about votes, anyway.

No, that isn't what I said nor what I meant. The thing that might or might not be very bad is doing business with such a criminal organization, not the existence or the activities of the organization (which uncontroversially are almost certainly very bad things).

These organizations can only exist as long as there are people doing business with them.

Could they? I mean, obviously anyone can argue anything, but what's relevant here is whether they could actually demonstrate that their benefit outweighs the marginal harm done. For that to be true in the case of a paedophile visiting a child prostitute, for instance, the relevant question would be: Has the paedophile's extra pleasure exceeded the child's extra suffering?

Well, the paedophile could argue that the child hooker in the streets of Bangkok is going to remain an hooker whether he visits him/her or not. After all, he is only displacing another customer, who, as far as he knows, could treat the child prostitute worse than he would. Even there is no other customer on that particular day, the life of the child prostitute isn't going to become noticeably different on the margin.
Does this make the visiting the child prostitute morally justifiable?

On LW, however, the audience is sufficiently clear-thinking that I am not worried that many people will jump to that wrong conclusion, which means you just get to look like someone who's trying to pull a sleazy rhetorical move. Which is probably why you're getting downvoted.

I think you have an over optimistic opinion of the audience here. People just tend to up-vote things that confirm their beliefs and down-vote things that challenge them.

A more productive (and, on LW, probably more effective) approach is to avoid such hot-button topics rather than embracing them -- or (if they're genuinely essential to the argument you're making) to distinguish clearly between asking "why doesn't your argument justify X?" and insinuating that your discussion partner actually does approve of X.

I didn't insinuate that people who are making "marginal ethics" arguments here are paedophiles who visit child prostitutes. I made a reductio ad absurdum argument to show that marginal ethics can lead to absurd ethical positions, at least in the opinion of those who believe that visiting child prostitutes is immoral.