Well I think when we say "play" boy, the term "play" here refers to a state of enjoyment. So a play boy is a boy that enjoys playing. Given the fact that the term "play boy" is situated in a contextual meaning gives it a very bad sense. This is to say when we refer to a boy as a play boy, where are asserting that this boy is cheating on his or her girl or boyfriend in a way that he is enjoying it. So a cheater who is not loyal to anyone except his to his lust and sexual desires.