Prostitution is a crime in 49 states. It would be perfectly reasonable for a government employees to be disciplined or terminated if they did commit such a crime. However, the agents involved in the recent so-called scandal were NOT in any of those states when this incident occurred. Prostitution is not illegal under federal law, nor is it illegal under Colombian law.
The only argument that I have heard so far for firing these agents is they could be subject to blackmail. But that same argument was used to discriminate against gay men in law enforcement and the military. When a unmarried federal employee visits a prostitute in a place where it is legal, it would be my guess that he's more worried about losing his job than social ostracism. The fact that he could face termination is the one thing that makes him vulnerable to blackmail.
It should be noted that married men routinely lose their security clearances when they have an affair. Secret Service agents do have a tremendous responsibility and must be held to a higher standard of behavior than everyone else, and so do the men that they protect. I propose that Secret Service agents be held to the same standard as the President when it comes to morals, and vice-versa.