Oh no, what an awkward thing to see on your Dad's phone @cjkb 
The info you've Googled is accurate. Lots of men occasionally watch porn online, in fact I'd go as far as to say that most men do - even married men. A lot of women do too. I understand how gross and creepy it is to think of your Dad doing that, but I think you handled it really well. You didn't embarrass him in front of the rest of your family, but you were able to let him know what you saw in a mature way.
I understand the desire to want to scrub that from your memory. I heard my parents doing it once and I wished I had some ear bleach! Unfortunately what's done is done, and now you just have to move on as best you can. I don't think it has to change your relationship with your Dad. If you continue to feel uncomfortable as time goes on, you can have a chat with him about it - maybe even laugh about it some day!