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.
There are definitely different ways to interpret their relationship and the play as a whole, for that matter, but I don't think there is much to insinuate a sexual relationship despite the kiss. Beckett would probably tell us that it is absurd to try to find much meaning in this play or ANYTHING else for that matter. There are a few ways that you can look at their relationship that doesn't involve a master/servant relationship OR a sexual one.
For one, it is hinted that perhaps Hamm is Clov's father or adopted father. This might better explain the kiss than physical desire.
Another way to look at their dynamic is to view them as symbols, even though they try to defy that view. In one interpretation, Hamm could be likened to the ego or brain of an aging individual while Clov could be the senses and faculties of the same individual. (he is Hamm's only connection to the outside world, without Clov, he is utterly isolated inside their skull of a room).
There are too many various interpretations to list here, but perhaps this can get you thinking.