I’ve just finished reading Brooke Magnanti’s The Sex Myth: Why Everything We’re Told is Wrong (review later) and she makes a useful point about pornography and obscenity.

The word ‘pornography’ comes from Greek roots: porno-, related to prostitution; graphos, to write. Stories about hookers, in other words … People in the nineteenth century became more worried about drawing a line between what was art and what was obscene. Those worries helped shape the view of what today is labelled ‘pornography’ versus what is labelled ‘erotica’ – even though few people, if any, can give a clear idea of the difference.
‘Obscenity’, meanwhile, comes from the Latin obscenus, meaning repulsive or detestable. Something obscene is something that is offensive to the morality of the time, something taboo. The definition of obscenity is different in different cultures, and even people in the same culture can disagree about what is obscene. Many laws have tried to define obscenity. While erotic imagery can be defined as obscene, it isn’t always considered so, and some laws recognise this

To which I would like to add the word ‘prostitute’: one who engages in sexual activity in exchange for money (payment).
Put that lot together and it means my world view goes something like this …
Technically pornography is stories about those who engage in sex for money. To me this means that any video (or other medium) which portrays a sexual act, where one can reasonably expect that (some of) the participants have been paid is pornography and (depending on one’s predilections) may also be erotic. Mere photographs of vulvas or penises may also be erotic, but are not a sexual act so are not (at least in my world view) pornographic; they aren’t ipso facto a sex act.
Whether one defines the pornographic, or the erotic, as obscene depends very much on one’s personal morality. We each have our own moral code, which may or may not align with that of society at large, and an act (image, description) doesn’t become obscene until it offends our morals and transgresses the line into being taboo. And that act doesn’t have to be sexual.
To use my own views as an example, I have no problem with the depiction of sexual acts, let alone the depiction of breasts, vulvas or penises. Pornography (as defined above) for me only becomes obscene when it crosses the boundary into being violent, non-consensual or involving minors or animals. There are sexual acts I greatly dislike (eg. male homosexuality), but that doesn’t per se make them obscene. But I do find many other things in this world obscene, amongst them the gratuitous killing of people and animals, blatant disregard for human rights, FGM, rape (of people and the environment), corporate greed and bankers mega-bonuses. YMMV.
So pornography is essentially, technically, amenable to definition. Obscenity is not readily definable so easily in anything other that one’s personal world view. Pornography is (should be) a largely objective measure. Obscenity can only ever be subjective. Which, of course, doesn’t mean that legislation cannot prohibit certain acts because the moral view of the majority of the legislature is that they are obscene for them – that’s how our collective, social, morality works and it is only by iconoclasts like me pushing the boundaries that such collective views are shifted.