I'll phrase it this way: Could I not watch an atheist YT video because there might be religious people on the bus? Could I not watch a video with swearing because there might be kids on the bus? Could I not watch a freakin Gordon Ramsay video because he might be eating steak and there might be someone just as disgusted by that as they are with porn?

As long as the guy wasn't actually jacking off, there's nothing all that wrong. Now, I don't particularly want to be unwillingly watching porn over a guy's shoulder on the bus, but it's his choice. I also disagree with people saying you're TA for calling him out publicly. If he watches porn publicly, you can call him out publicly. He obviously doesn't mind people knowing what he's doing.

Honestly, NAH, cuz he can watch porn if he wants, and all other reactions following from that we're also completely warranted.