cheating??

I was on my boyfriends phone the other night (we trust eachother I go on his phone and he goes on mine) I went into his google and what came up was a porn website which was the last thing he went into his browser for, as soon as I saw it my heart kind of dipped, i felt like he shouldn't have done that, it makes me feel like I'm not good enough that he went on a porn website and masturbated over another woman? I spoke to him about it and he saw the effect it had on me, and promised me he wouldn't go on any porn websites again, I don't know why it bothered me so much, is it okay I felt like that? Should I just learn to live with it... Idk it feels weird that my partner got off to another woman's naked body