"Sex Trafficking" is just new branding for prostitution. It was conjured up by a coalition of feminist activists in collaboration with hard-right Christian organizations as a new boogeyman because of data that says most US citizens are not as outraged by the concept of prostitution as they once were.
Sex Trafficking nicely conflates Child Abuse, Slavery and Prostitution all in one vague phrase and is impossible to criticize the concept politically. So it is having the result its creators hoped for.
So now any man who hires a prostitute is a sex trafficker, which the associated stigma.
There are several reasons why a large cross-section of society are reflexively against prostitution. The reasons vary from economic to political.
Prostitution arrests, like Marijuana Arrests, are a major source of revenue for the Govt. It is sort of like Marijuana. Legalizing this would result in considerable revenue loss.
A legal and safe avenue to have sex whenever they want till screw up the age-old power-balance between men and women. A very large reason for why men put up with the risks and hassles of marriage is because men are helpless against the force of their sexual drive between the ages of about 14 and 50. This particular weakness has been cynically and ruthlessly exploited from time immemorial by governments and women. If men do not invest in long-term commitment, a large source of productivity will disappear... as they will do the bare minimum to survive and satisfy their direct wants and not worry about putting up with shitty jobs to save for their kids' college funds.
And of course, there is the over-powering man-hate from feminist organizations, powered by women who cannot stomach the thought that there may be some man in the world who may not be completely unhappy.