Honolulu police officers say they need the legal protection to catch lawbreakers in the act.
Expert Derek Marsh says the exemption is "antiquated at best" and that police can easily do without it.
"It doesn't help your case, and at worst you further traumatize someone. And do you think he or she is going to trust a cop again?" asked Marsh, who trains California police in best practices on human trafficking cases and twice has testified to Congress about the issue.
A Hawaii bill cracking down on prostitution (HB 1926) was originally written to scrap the sex exemption for officers on duty. It was amended to restore that protection after police testimony. .
During recent testimony, Honolulu police said the sex exemption protects investigations and should remain in place.
"The procedures and conduct of the undercover officers are regulated by department rules, which by nature have to be confidential," Honolulu Police Maj. Jerry Inouye told the House Judiciary Committee. "Because if prostitution suspects, pimps and other people are privy to that information, they're going to know exactly how far the undercover officer can and cannot go."
Skeptics, such as Roger Young, a retired special agent who for more than 20 years worked sex crimes for the FBI from Las Vegas and has trained vice squads around the country, remain unconvinced.
Young said Thursday, "I don't know of any state or federal law that allows any law enforcement officer undercover to penetrate or do what this law is allowing."