I think the central issue here is that in the absence of sound, by watching somebody watching porn you're trespassing on their private activity. Sound makes a difference because with sound you're inflicting it on everyone within earshot, but otherwise if somebody next to you is (silently) watching porn and you're watching it/them watching it, you're visually eavesdropping (so to speak) on their activity, to which the answer surely ought to be look elsewhere. Lots of people complain that with electronic gadgets such as smartphones and ipads etc. people on public transport are all locked into their own little bubble. Maybe so, but you can't have it both ways - you can't complain about that and at the same time moan that they're watching something you don't want to see. You have no reasonable ground (IMO) to complain that somebody is watching sonething that they want to watch but which you find objectionable - if you are watching it over somebody's shoulder you are, again, committing the visual equivalent of eavesdropping.