I used to think that outlawing prostitution was irrational, but I'm starting to reconsider because of the information coming out in recent decades about the negative long-term psychological effects on women who have multiple sex partners, which I think can have damaging effects on society overall.

I agree that trafficking is almost certainly a business that transports willing passengers to destinations they want to enter illegally. What you say about the UK situation reminds me of a case in Florida some years ago. A work camp full of illegal aliens was busted. The two women among them (who didn't pick produce, but serviced the men) claimed to be sex trafficking victims. I was sure this was nothing but a bid for leniency.

I think the outrage being more focused on sex trafficking than kidnapping or forced labor is that there really isn't anything an ordinary citizen can do other than be outraged, and it's useful for demanding new legislation (the Mann Act forbidding transporting women across state lines in the US was the fruit of a moral panic including claims that 50,000 women per year were being abducted into prostitution). It's a lot easier to generate and shape public outrage over mostly-imaginary crimes than it is to do the hard things like campaign against the actual slave markets that sprung up in Libya after Khadaffi was killed.