For those who are saying they don't see the problem with porn, remember that most talking here are from the US - we have puritanical rules from the foundation of our society that says violence is okay but sexual intercourse is bad. After all, seeing a guy get his face smashed in or buildings explode is okay for kids but sex is not.
However, I do have to agree with the argument that a plane is a public space. No one says you can't view your porn in private (say, maybe in the bathroom while using headphones if you really must while on a flight - but if you can't go without porn that long, you may want to seek some help with your addiction). What they are saying is that a certain degree of public decorum should be maintained.
By their same argument, I think violent movies should also be blocked - but I doubt there would be any support for that.
More to the issue, however, is that the argument is inane. Those who would view porn via in-flight wi-fi would most likely have alternate sources if blocked. Files on a computer or thumb drive, porn magazine, DVD, CD, or steamy novel. Blocking wi-fi will not block porn, plain and simple. (Kinda like how people say EA's DRM is pointless because it doesn't affect those that pirate the game.)