I don't know what's more offensive about the concept of obscenity - the idea that a form of speech could be beyond first amendment protection because it's offensive (isn't that the point of first amendment protection?), or how totally subjective the test for obscenity is.
A picture like this one above, is probably not offensive enough for anyone to seriously call it obscenity (at least by legal standards - and I should hope not), but the fact remains, that sexual speech is specifically marked out for consideration of obscenity (if that doesn't reflect moral values - something the government is not supposed to cater to - I don't know what does), and it is, by definition, a matter of opinion whether a certain instance of speech is defensible or not.
You may say that the majority rules, and that may be a valid argument in the case of open, public display, but what private individuals pass willingly between themselves is not a situation that warrants government intervention.
Now, if there were such a thing as violent obscenity - where lawmakers went out tracking down explicitly violent materials that were "patently offensive" and lacking in socially redeeming value and all that, at least things would make slightly more sense. But we glorify violence in this culture, while simultaneously treating sex like a cancer on society.
What business of it is yours if somebody is deriving pleasure privately in a way that offends you? Is that really a good reason to throw a person in jail?