Boyfriend of three years messaged a girl he knows for nudes

I went on my boyfriend’s Reddit account (it’s not hidden he told me the username) and I saw he left a comment on a porn subreddit.

I click into it to see it’s of a naked girl he went to highschool with! He’s begging her to post more and that she’s his “favorite”.

He’s talked about this girl in the past and they follow each other on everything and stay in touch. I’ve always gotten an strange feeling about his feeling towards her - he was always too critical of everything she did, like he wanted to show me he didn’t like her.

Now I don’t know what to do. Part of me wants to snoop through his phone because I don’t trust him now but the other part of me is like will I ever trust him again?

I honestly don’t feel anything but I know I should be upset. I’m having a hard time of thinking of an appropriate response.