The reason for this probably stems from the fact that the upper classes had more time to spend in the palaestra and gymnasium where men interacted with men exclusively. Young upper class boys spent as much as three quarters of their time in the palaestra, which meant they would spend the majority of their time with other men, older and younger, all of whom were nude for the most part. Male homoerotic relationships most commonly denote a man of high social standing engaging in the courtship of a young boy, who was often a member of the upper class as well. The younger boy could also be a slave though. It was not impossible for two men, or two boys of lower classes to have homosexual relations, but there was a law forbidding a male slave from having this type of relationship with a young boy of high social standing.
Vases tend only to depict scenes only of upper-class courtships however.
6 Slaves were also forbidden from anointing or undressing in the palaestra. Since a large number of vases depicting homoerotic scenes use references to the palaestra as a setting, it makes it even more unlikely that a slave would ever be depicted on a vase. There were certain customs and rules that went along with such courtships, which are well illustrated in a number of vase paintings. As suggested above, homoerotic scenes are usually depicted between a man and a boy. The pair is usually of a similar freeborn citizen status.