Most likely broken commandments: 2, 3 and 6
2 - "Thou shalt not take the name of the Lord thy God in vain." - Likely in the throes of sex there might be some "Oh God!" to head.
3 - "Remember to keep holy the Sabbath day." - It is doubtful courtesans would not work on Sundays.
6 - "Thou shalt not commit adultery." - No need to comment THAT one.
Honorable mention: 9 - "Thou shalt not covet thy neighbor’s wife." - Whilst a courtesan will probably not be guilty often of this transgression, she would likely incite the transgression on part of their clients, even if commandment 6 is not broken in the process.