Horror Porn

Why is porn considered obscene but people getting their heads chopped off only mildly offensive? I find this really weird since in real life people actually do have crazy sex, but are rarely allowed to kill someone with an axe and eat their remains in real life. Doesn't it seem odd that being a porn star is considered shameful, while being a movie villain or serial killer is sometimes considered sexy i.e. DEXTER? You would think it would be the opposite --> That porn stars would be accepted by the mainstream and actors who chose to play mass murderers would be treated as pariahs. I think that porn stars deserve more respect. They're job isn't nearly as glamorous, they get paid less, and get treated like fallen women (or men) while movie stars get treated like we're lucky to be allowed to view them and get paid more for an easier job
That's not to say porn stars have it that tough, I'm just saying society is a bit judgemental when it comes to the people who star in dirty movies.