I (speculate) that it has something to do with it being a Supernormal Stimulus.
A picture of people can be made more and more pornographic by makeup and poses and clothing which accentuates the desirable sexual characteristics. Even more by careful use of lighting and camera angles. Again more, by surgical enhancement. More and more by image editing afterwards, and by the presentation, until the picture has gone from interesting to eye-catching and attention arresting and driving the viewer wild.
Similarly, photos of the planet are not Earth-porn, not even if they're pretty. But if they're an extreme place (cliffs, desert, mountains), taken at an unusual time (dawn, dusk, solar event), taken with High Dynamic Range image processing, with careful composition, image editing ... then they take the "nice picture" response right the way up to "wow!".
e.g. this is not earth-porn:
But this is:
Even though they are both fields. I think it's that deliberate raising and raising of wow factors all the way to maximum which differentiates between classic foo-enthusiasm and foo-porn.
The focus is no longer on the content for itself - where the field is, who uses it, what techniques they use for farming, how they protect crops from birds, who was on holiday when they took the picture - the kind of things an enthusiast of 'foo' might be interested in. The focus is now on the WOW factor of the picture, the content could be substituted for any content without it mattering. The content is less important. Gun enthusiasts care about guns, gun-porn cares about making the viewer think 'wow'. Train enthusiasts care about trains, train-porn cares about making the viewer think 'wow'.
And so porn is the first, most common, most popular thing which used that technique, so the name comes from there, and the more extreme more taboo, rude, term wins out over any normal everyday term like 'processed pictures' or 'nice pictures' or whatever.
2foo is an all-purpose variable that can contain anything. It is roughly synonymous with "universal placeholder". – Tᴚoɯɐuo – 2017-05-31T10:51:21.413
31It's also worth noting that this usage is not common, and is primarily found among Internet communities, or perhaps even just Reddit. If you say to an average person on the street "Take a look at this Food Porn" they'd not necessarily know what you meant. – Muzer – 2017-05-31T16:02:12.430
16@Muzer I don't think that's true at all, based on my experience, "food porn" is in common usage even among older generations who don't use online forums. – Jack M – 2017-05-31T17:08:57.243
10Personal anecdote: many years ago in the elder days of the Internet a colleague and I came across a web site devoted to pictures of absurdly high calorie food. After we had spent several minutes staring in wonder at various dishes I turned to my colleague and said "You realise this is food porn, don't you". I'd never heard the term before, it just seemed an obvious description. – Paul Johnson – 2017-05-31T18:04:25.693
3All the answers and comments so far describe it as something pleasant/idealized/fetishized, but isn't it mostly a derogatory term? Most of our literature, cinema and other noble arts revolve around sex, almost never directly depicting the sexual act itself. Remove all the subtlety and embellishment and give us a close-up of banging genitalia - that's porn, a shameful pleasure of unwashed masses. Similarly, a lot of our life revolves around food - remove everything but give us a close-up of a dish, that's food porn. Etc. Did I get it wrong? – Headcrab – 2017-06-01T06:13:48.367
1@Headcrab "Did I get it wrong?" Possibly... Porn (in the traditional sense) tends to be used derogatorily by people who don't like/approve of it to describe what other people (who see themselves as having a "healthy interest in sex") like to watch -- a lot depends on one's point-of-view. The foo-porn usage hangs more off the "like to watch" part than the derogatory part. It's almost a case of an irregular noun: "I appreciate artistic nudes", "You watch smut" and "He/she/it devours porn" :-) – TripeHound – 2017-06-01T06:56:07.233
1Bear in mind that this usage is also NOT universally accepted and there exists an corresponding subreddit /r/NoSillySuffix dedicated solely to removing all mention of porn from these sites. – Chris Petheram – 2017-06-01T08:44:22.413
@Headcrab I don't think so. Someone might say "I spent an hour looking at food porn and now I'm starving"... Generally these images are extremely beautiful, well-crafted food... perfectly plated and drool-worthy. – Catija – 2017-06-01T23:49:33.827
@Headcrab - I think this borrowing of the term “porn” is more sarcastic than derogatory. I don’t think that “food porn” or “puppy porn” carries the same negative baggage that “smut porn” would. – J.R. – 2017-06-03T01:01:27.923
Umm, "food porn" is an acceptable expression, but please never use the phrase "puppy porn". – Dawood ibn Kareem – 2017-06-03T04:10:57.890
@DawoodibnKareem I'm curious what does "puppy porn" mean; it sounds adorable! Also, I guess I should never use "pussy porn" either :/ – nalzok – 2017-06-03T04:50:10.913
@SunQingyao, "puppy porn" has two possible meanings: cute pictures of puppies, and pictures of sexual activities involving puppies. So it kind of slides back to the original meaning of the word "porn". This creates a mental dissonance which the listener will want to resolve. – Paul Johnson – 2017-06-03T21:10:24.160