This is completely absurd. If a woman thinks that she is the only woman their significant other fantasizes about, then she is totally deluded. He's picturing her naked. He wants to bone her brains out. He'd fancy a shag with her, too.
Pornography serves a purpose.
Men are visual creatures. We are basically no better than animals, and in the animal kingdom, males are always looking for a suitable mate. They are biologically programmed to want to spread their genes to as many as they can, as often as they can. At least he isn't dicking the next door neighbor. Besides, porn is kinda cool.