Is it silly for me to think of my husband looking at porn as a form of cheating on me?

"Cheating you" maybe. Or he could be "cheating on you". Either way it doesn't bode well for the marriage. It seems you are not in favor of him doing that, for good reason. Your connection with him is likely to go down hill. It helps impotence in men sometimes. Some people do use porn as a way of helping sexual relations but it is not common and is by mutual consent. Without that consent, cheating is a good word for it.
No. Many women have felt cheated on when her husband looks at porn. Looking at the porn industrys "perfect" image of the super thin, sexually insatiable, young, female having no needs of her own makes many women feel upset, question their own bodies, not feel good enough and feel cheated on. Not silly. Normal. Take care.
YES/NO. Using the term, "silly" implies a judgement, which i'd always avoid. If it makes you feel hurt that your husband is interested in viewing other women, no one can take that feeling away from you. Pragmatically, it is not really cheating if he's told you about it (or doesn't hide it) and it sure beats him going out, behind your back, to watch or engage with other women.
Maybe/maybe not. Having an active fantasy life is usually a good thing. Many men like porn but realize it's not as satisfying or valuable as a real relationship, so you're not necessarily competing with the women in his fantasies. His interest in porn doesn't mean he finds you unattractive. You're the real woman in his life who cares about him and those other women, though they look perfect, are unattainable.
Big picture. Essentially all men seek out erotic images from time to time. Tastes vary tremendously, just as does everything else involving sexuality. It has nothing to do with whether he loves you. Most men get bored quickly, realizing this is a fantasy world and dead end. In your bedroom, be adventurous and work hard to keep each other happy; you may find images you enjoy viewing together.