Do I have a right to be upset?

My husband and I have had the talk about porn and its role in our relationship: on our own time when the other person isn’t around, go for it, but when the other person is around, don’t. This has been a great arrangement for us for the past four years of our relationship.

Tonight he fell asleep with his phone in his hand, unlocked. The screen was still lit up (his hand was still touching the screen), so after a while, I picked up his phone to lock it and put it on the charger (we do this for each other whenever we need). I glanced at the screen, and it’s open on a subreddit dedicated to NSFW pictures of plus size women (note that I am plus size as well). This includes pictures of vaginas, boobs, asses, etc.

I feel like I’ve been cheated on. Not necessarily to that degree, but I imagine it’s close. He didn’t try to initiate any kind of sexual/sensual interaction today, just went straight to looking at other women. Is it okay that I’m upset? What do I do? Should I suck it up? Please help me figure this out.