It would have been downright unheard of for ancient or medieval armies to actually hire sex workers as an expense. If any bodily entrepreneurs felt like following the army, that was their own business. Soldiers are generally young men, and at least in the Roman military, enlisted men were officially forbidden from marrying until they finished their term of service.

Most armies, right up until the First World War, were accompanied by large numbers of "camp followers" who often included the wives and concubines of the soldiers -- as well as good number of prostitutes, although these women often only performed sex work on the side -- their main income was doing washing, sewing, cooking, and other domestic tasks for the soldiers. Before the 18th or 19th centuries, most armies did not have centralized procurement and provisioning systems -- often the soldiers would be expected to pay for their food, uniforms, etc, out of their regular wages. Some times the soldiers would be provided a grain ration or food stipend, but left to their own devices in terms of food preparation. I'm not really an expert on the military side of things though, so I can't provide an more specific examples -- plus, my own research was only focused on the Early Modern period, so I'm not at all certain of the specifics of ancient or medieval military forces.

According to several eyewitnesses, Joan of Arc (c. 1412-1431) used the back of her famous sword to chase prostitutes from the French army. Here's an imagined image of her at work from the [Les Vigilles de Charles VII] (http://www.maidofheaven.com/joanofarc_vigiles_charlesvii.asp), a 1483 poem about her. She also forbade soldiers to swear (good luck with both of these efforts). The presence of prostitutes to chase suggests they were a usual side business of armies, "camp followers," as TectonicWafer" notes.

Prostitution was often regulated in urban environments, even in the middle ages, though it was much more closely regulated in the early modern period. I'm by no means an expert on prostitution, but it seems incredibly likely that the camp followers mentioned by /u/TectonicWafer included prostitutes.

Here are some books for further reading:

(1) Ruth Mazo Karras. Common women : prostitution and sexuality in Medieval England (1996).

(2) Katherine Lewis. Young medieval women (1999)

(3) James Brundage. Sexual practices & the medieval church (1982)

(4) Andrew McCall. The medieval underworld

(5) Barbara Hanawalt. Living dangerously : on the margins in Medieval and early modern Europe (2007)

(6) Leah Lydia Otis. Prostitution in Medieval Society : the History of an Urban Institution in Languedoc. (1985)

(7) John Baldwin. The language of sex : five voices from northern France around 1200 (1994)

Hope this helps a little. Happy Reading!