Is it normal for my boyfriend to sneak away from me to watch porn?

It's mostly when I am sleeping, and he wakes up before me in the morning. He takes the utmost care to open and close the bedroom door silently, but that doesn't really matter because I am a light sleeper. I'll wake up after he leaves, and then just wait for him to come back.
Sometimes, he'll go in the bathroom or another room with his iPod and lock the door, and he'll be in there for a half hour or more at a time. And other times, he might even take his iPod and go outside to lay in the back of his car. (If you're curious about how this works, we live in a rural area, so there aren't very many people around. And his car has tinted windows.) He thinks I don't know he is masturbating, but it's obvious when he comes back with a rapid heartbeat, deep breathing, sweating, and he always changes his pants immediately afterward.
Is it normal for him to sneak away from me like this, to relieve his sexual tension? If he really needed to, he could wake me up to have sex, but he doesn't. It's not that our sex life is bad. In fact, it's wonderful. I just wish he would only resort to pornography if he has a strong urge while I am not home.