1. He has been brainwashed... by an extensive, multi-faceted, multi-billion dollar corporation with more revenue and more resources than the NFL, NBA and MLB combined (Source:
CNN.com). This industry has been pouring a great deal of effort into a marketing campaign that seeks to make pornography more acceptable, more mainstream and yet more anonymous. In other words, they are putting every effort into making your significant other think watching pornography is not only normal, but something he needs.
2. He thinks you're wrong to dispute the untold powers of the pornography industry.
You try talking to someone who has been brainwashed, even in something unrelated to pornography. They have been told over and over since they were a child by the media, their friends and even their parents, in one way or another, that something is acceptable. The average age of exposure to pornography is now eleven years old (Source: Inadvertent Exposure to Pornography on the Internet, page 5). Having been brainwashed into thinking this is normal behavior since the tender young age of eleven, it is very difficult to give it up. The "normality" of pornography is nothing short of indoctrination during the formative years of his life. If you tell someone the same thing over enough at this age, not only will he come to believe it, but it will eventually become virtually inseparable from his identity.
3. Peer pressure. His friends, from whom he seeks advice, have talked with him about the issue. His friends, also brainwashed since youth by the same industry and also probably struggling with the same addiction, have told him that it is not his problem but that of his significant other. On an unrelated note, an additional question might be, "Why do they lie about it?" Again, peer pressure, and often following a friend's advice.