Well it's not the only plausible reason even though it's by far the most realistic.
Some apps save photos and video by default, so it's not beyond the realms of possibility it was sent to him by someone else, he's opened it to look at it, then either forgot to close it properly, or opened the same app/his saved items and accessed it for a second time through clumsiness.
I'm being extremely generous to him here as I think it's overwhelmingly likely the correct answer is the obvious one, I'm just pointing out that you can not conclude that the only way this could have happened is for Parish to have wilfully gone surfing the internet looking for porn. There are other possibilities, no matter how improbable, so whether you, I, or anyone else has ever 'accidentally opened porn on the internet' is not particularly relevant until such time he claims that is indeed what happened.
FWIW I certainly have, but it only really used to happen when I was accessing specific sites that I knew had poor security, and only back when browsers and OS didn't have the sort of pop-up killing tech and firewalls that they do now. I absolutely knew what I was getting myself into, so it's not the sort of thing I would ever have attempted on a device in a public place.