It's no secret that lots of people watch pornography on the internet. It's usually something done behind closed doors - but how would you feel about someone watching porn in public? The BBC's Siobhann Tighe describes a troubling experience on a London bus.
It had been a long day at work. I got on the bus at 7.30 in the evening and it was cold and drizzly. All the passengers were wrapped up in thick coats, hoods and hats.
Inside, the bus was softly lit and I was expecting to zone out on my way back home: just let the day go and switch off.
I sat on the lower deck beside a complete stranger and didn't give it a second thought. I was just relieved to get a seat. As we meandered through the London traffic, my gaze was drawn to my neighbour's phone. I wasn't being nosy but in the dim light of the bus, the brightness of his mobile caught my attention even though he was slanting it slightly away from me.
Although I didn't mean to or want to, I found myself looking over towards his mobile a few times and then it suddenly occurred to me what was going on. The man beside me was watching porn.
Once I realised, although I genuinely didn't mean to, my eyes kept on being pulled back to it. I couldn't quite believe it. First he was watching animated porn, with the two naked characters in lurid colours repeating their movements over and over again. Then he started watching a film, which seemed to begin in a petrol station with a large woman in a low-cut yellow top and blonde hair peering into the driver's window.
I didn't hear any sound, apart from a brief few seconds when my fellow passenger pulled the headphone jack out of his mobile, and then reinserted it.
The man didn't seem to notice my glances towards his phone, maybe because his hood was hampering his peripheral vision. He seemed oblivious to me and others around him, who admittedly wouldn't have been able to see what I saw.
We eventually arrived at his bus stop and because he had the window seat and I had the aisle, he made a motion that he needed to get out, and he muttered a "thank you" as he squeezed past me. I watched him get off and walk down the street.
I felt uncomfortable and annoyed, but I didn't do anything about it. I didn't say anything to him and neither did he pick up on any of my glances or quizzical looks. His eyes didn't meet mine so I couldn't even communicate my feelings non-verbally and it didn't occur to me to tell the driver. Even if I wanted to, it would have been difficult to get to the front of the bus because it was packed.
But when I got off, questions flooded into my mind about what I had just experienced. What if a child saw that? Are there any laws about looking at porn in public spaces? If there are laws, how easy are they to enforce? Why did this passenger feel public transport was an appropriate place to watch porn, and should I be worried from a safety point of view?
As a journalist, I also looked at it from his point of view, even though he made me feel uncomfortable. I asked myself: is he within his rights to look at porn on his private device wherever he is? Do civil liberties in our society grant him that freedom?
But in my heart, I was offended.
When I mentioned it to friends, everyone seemed to have a story of their own, or an opinion.
"It happened to me when I was with my son having a coffee at a Swiss airport," one said. "Two Italian guys were sitting next to me. I said something because I felt safe and I sensed there'd be support if an argument ensued." It worked, and they politely switched the laptop off.
It certainly got everyone talking, but like me, no-one was sure where the law stood.
According to Prof Clare McGlynn from Durham University who specialises in the law around porn, there's little to stop someone viewing pornographic material in public - on public transport, in a library, in a park or a cafe, for example.
"It's like reading a book," she says. "They are viewing lawful material which is freely available, and restricting people's access to it presents other challenges."
In Prof McGlynn's view, the law would only prevent it if the porn viewer is harassing someone or causing a disturbance.
So, what do you do? Prof McGlynn describes it as a dilemma.
"It's like someone shouting at you, calling to you to 'Cheer up, love!'" says Prof McGlynn. "Do you confront it, or do you put your head down and walk along?"
But when I contacted Transport for London, they appeared to take the case very seriously.
"If someone has made you feel uncomfortable, for example by viewing pornographic material, please tell the police or a member of our staff," I was told.
A member of staff said passengers should report incidents like to this to the bus driver, who would tell the control centre, and the information would then be passed to the police for them to investigate.
In Prof McGlynn's view, there is not much the police could do. On the other hand, James Turner QC contacted the BBC to say that there is a law - the Indecent Displays (Control) Act - which might form the basis for a prosecution.
Five years ago, in the US, the executive director of a group called Morality in the Media had an experience similar to mine on an aeroplane. As a result, the group - now called the National Center On Sexual Exploitation (NCOSE) - campaigned to get the major US airlines to stop passengers watching porn.
"All of them except for one agreed to improve their policies to prohibit passengers from viewing this material during flights and agreed to better train their flight attendants on what to do," Haley Halverson of NCOSE told me.
Buses don't have flight attendants, though. Nor do trains. And even if police wanted to investigate incidents of porn-watching on public transport, passengers can get off whenever they like.
How would officers catch them and question them then?
Siobhann Tighe and Prof Clare McGlynn spoke to Jenni Murray on Woman's Hour, on BBC Radio 4. Listen to the discussion here.