As natural as women reading romance novels or watching chick-flicks is. Men are expected to know what they are doing in the bedroom (granted, porn isn't usually the best teacher of that). Men are very stimulated visually. Men's initial attraction is almost always physical. and so on.
So, yes it is relatively natural for men to be stimulated by porn and desire it's stimulated from time to time. You have to remember that men do not drop an egg once a month, or have a monthly hormonal cycle. Men produce sperm all day/every day. We need regulate stimulation to get it out (or it will eventually come out while we sleep...very messy). Men usually prefer a partner for this, but sometimes we may desire variety (without cheating), or decide that it isn't always worth the effort to initial sex with a partner (when we just want to use them for a release, which women understandably feel used if you do too often).
So, yes it is natural for a man to bring his sexual desires to his partner (assuming he has and wants one). But it is also natural for a man to desire sex far more often than a woman, and frequently just the sex (which most women need to build up to to enjoy). Porn provides visual stimulation for him to obtain release, variety to satisfy his curiosity, etc without any effort or work. The sign that there is a problem is when a man prefers porn/masturbation to his partner. Then either something is wrong with the relationship (at least sexually). With married couples too often it is because a woman has turned down a man frequently, so he has shutdown and stopped being attracted to her physically. He may actually still love her, but his body has been conditioned to no longer be physically attracted to her (go to the well and find it dry too many times, you stop going to that well when you are thirsty). And once physical attraction is lost for a particular woman, it is very hard (if not impossible) to get it back for most men.
So, porn is fine as long as you don't stop receiving him when he come to you (yes you can make reasonable conditions), or he doesn't stop coming to you, then I wouldn't worry about porn.