It could come down to a needs vs wants arguements, food is one of the basic human needs along with water, shelter and so on. Porn is just a general passtime, hobby or form of entertainment for people (sex for reproductive purposes is different from general pornography itself.....though i'm sure there is a fetish out there of people getting off on birthing videos)
While granted unhealthy food over a long period is very bad for you physically(also mentally possibly in the sense that your starving your brain of good/vital nutritian i suppose) compared to pornography it is a basic human need for survival.
Now, if we talk first world for a moment then i could imagine it actually being regulated/banned or whatever. Food is more than plentifull, we already so many health issues as nations which this type of food has caused(Though i suppose there are fatties who got fat eating healthy food....now that would be just lazy though)
Though given populations keep increasing and it's predicted in X or so years when we reach a certain point in population, then first world nations will actually be struggling to fill their food needs to.(which could bring up another argement if banning unhealthy food now would even have a point)
Porn itself is generally seen to be harmfull to alot of thing, first being "the children" in which i can agree to a certain age(like 12/13) kids really have no needs to be exposed to the issue.....though from there on personally i fail to see the damage it does when kids already get taught sex ed.(Seriously i could show people thousands of males in my generation who knew of porn from like 10 or so and are fine adults)
Porn is also argued to corrupt and/or devalue societies morals/values(say hello to the banning of "hardcore" porn in alot a few places for example....though in the UK that has more to do with a certain murder case) These arguements generally come from very prude like governments, old fashioned farts or your good old religous folks.
<Insert religous zealot reasons>
Probably some hypocrosy on the the part of "What i shove in my mouth is up to me" and "what kind of porn i watch is up to me" fronyt anyway i can atleast.