Is it normal to have sexy thoughts about your best friend?

I have this best friend that I've known since i was ten. We went through middle and high school together, and now we live together in college. But in high school, while we were in the locker room, I would look at him changing. He never saw me, so I kept on doing it. I even looked at him in the showers, and that is the reason i don't take a shower at school, because I get "hard". Now, in college, he just walks around naked in our dorm and he doesn't even care. I try not to look, but it's too hard.Is it normal?