Nobody is proposing that sex trafficking should be legalized. On the wider issue of prostitution, there is a fundamental difference between it and the crimes y

Nobody is proposing that sex trafficking should be legalized. On the wider issue of prostitution, there is a fundamental difference between it and the crimes you mentioned: murder, rape and kidnapping are always wrong, regardless of whether money is involved. On the other hand, most people don't have a problem with sex between consenting adults; they only have a problem with it if there is payment involved. This of course raises the question why an otherwise acceptable act becomes immoral when it is in exchange for money...