Boyfriend views adult content.

My boyfriend (19) told me (18) that he watches porn, and while I appreciate his honesty, it leaves me feeling really down. I truly love him and he treats me wonderfully, better than I ever expected. However, I'm struggling to understand why he still thinks about and looks at other women in that way; it makes me uncomfortable because I couldn’t imagine doing the same with another man. How can I bring this up with him in a way that’s open and understanding, without making him feel upset or accused? This is my first serious relationship, and I’m really afraid of ruining it.