Feelings about your other watching porn?

When we first started dating, my boyfriend told me that he never felt the need to watch porn anymore. During a long distance relationship, we used to send pictures to each other to masturbate since we weren't together for long periods of time. He moved right by me, but we continued to do that sometimes. As of two months ago whenever I would ask, he would say he didn't want to, or that it's hard for him to be horny unless I'm with him in person. I felt really sad and rejected every time that happened, so I finally just stopped asking. Now we've been together for a year, and Yesterday when I was with him, in the middle of doing sexual stuff, it came out that he's been masturbating a lot to porn for a while now. Ever since then it's been hurting so much, he has hundreds of pictures of me saved from all those previous times, but I dont know why he chose to watch porn of other women. Especially after rejecting me from doing that on Skype or pictures. I'm so upset by this and I dont know why it's bothering me so much. Does anyone have any thoughts?