Because he is blind and confined to a wheelchair, Hamm is completely dependent on Clov. In that sense, they are more than master and servant because without Clov, Hamm would be. Despite being terribly mistreated by Hamm, however, Clov does not leave him, not out of love or loyalty, but because he doesn't know the combination to the lock on the food pantry. Hamm does ask Clov for a kiss, but it may be from a need for any kind of human contact or affection rather than from sexual desire.
This is a very strange play, and as such, it is difficult to place "normal" rules of conduct on its characters. The play is absurd, the characters are absurd, their motivations and behaviors are absurd.