Prostitution
From Memory Alpha, the free Star Trek reference
Prostitution is the selling of sexual favors for money or other things of value. Someone who engages in such activity is known as a prostitute. The term "whore" was a derogatory term usually aimed at prostitutes.
Q revealed that if Quinn never inspired Sir Isaac Newton, Newton would have ended up in a prison as a suspect in several prostitute murders. (VOY: "Death Wish")
During Spock's youth he was often taunted by bullies in an effort to evoke an emotional response from him via insults. The bullies were finally successful on their thirty-fifth attempt when they called Spock's father a "traitor" because he married what they described as a "Human whore." (Star Trek)
The malevolent entity Redjac murdered numerous prostitutes in London from 1888 to 1891 during his time on Earth as Jack the Ripper. (TOS: "Wolf in the Fold")
In 2004 on Earth, a man named Loomis was kidnapping various people in Detroit, Michigan for their blood types. Among these people was a prostitute who had donated blood earlier so that they could check for sexually transmitted diseases. (ENT: "Carpenter Street")
While stranded in San Francisco of 1893, Data was offered the company of Lillian, a prostitute, by the bellboy of the Hotel Brian, Jack London. (TNG: "Time's Arrow")
Prostitution still existed on Farius Prime. In 2374, Bilby arranged for a Farian prostitute for Connelly, but he claimed to have already met a woman who "worked at a flower shop", paid the woman and had her leave. (DS9: "Honor Among Thieves")
Unofficially, Quark's on Deep Space 9 featured prostitution, in the form of holographic "entertainment." (DS9: "Meridian", "Profit and Lace")
In 2370, Kira Nerys posed as a prostitute to get past the security guards at the Hutet labor camp. (DS9: "The Homecoming")
In 2372, Captain Kathryn Janeway posed as a prostitute in order to gain access to the prison cell, where B'Elanna Torres and Tuvok were held captive by the Mokra Order. (VOY: "Resistance")
