The one I use is, "If sex work is work, and it were to be completely legalized, would you demand that homeless women in transitional housing programs that require them to get a job accept sex work, if they were unable to get any other form of employment? And if they refused, would you kick them out of the program?"
Sex work advocates who can blithely say, "Well, if it was their choice, no, I wouldn't mind if my (hypothetical*) kids did sex work!" will shut right up when you hit them with the inevitable consequences of the law treating sex work as just another job, no different than waiting tables or making coffee.
*Hypothetical kids, because very few of the "sex work is work!" types I've encountered actually have kids. The ones who do tend to resent them.