I'm curious what moral distinctions (if any) exist between, for example, the Venus de Milo or the Birth of Venus and "soft" pornography like Playboy?

Well I'm definitely no expert here, but often in our moral evaluations we take into account intention/motive (as perhaps one factor among others). And if one might arguably hold that "art" aims for some kind of "higher" purposes beyond merely sexual stimulation/titillation, then at least those works of art have some kind of additional value beyond their attractiveness (while, perhaps, the "soft porn" aims only for the stimulation....)

One of the formulation of Kant's Categorical Imperative is that we should never treat humanity as a mere means to an end. I wonder, then, whether this means that horror film directors are behaving unethically. After all, in a horror film, terrible things are made to happen to human beings, solely for the purpose of frightening/pleasing the audience. The human beings may be fictional, but it is nevertheless the fact that they are fictional *humans* that makes horror films effective (as opposed to a horror film where the victims are all robots). It seems to me that the humanity of the fictional characters is being used as a tool to manipulate the audience's emotions. Does this fall under the umbrella of the humanity formulation of the Categorical Imperative? If not, why not? It certainly seems that it is the humanity of the victims (including their emotions, their aspirations, their mortality, their ability to suffer) that is key to the function of horror films.

A very interesting question -- and while I know next to nothing about Kantian ethics, I might chip in here the observation that in a (clearly 'fictional' film) there is no particular, actual, individual human being who is being used as a 'means to an end' (unless of course the actors etc. are being exploited in some way by the director/producers etc...!). Perhaps humanity in some general way is being used, but no individual humans -- so I would imagine that the Kantian proscription wouldn't apply .... (Now if, in a film, the actors were representing actual particular individuals, even if in a fictional way -- like a highly fictionalized biopic, for example -- that might be a different story ....) hope that's useful- ap