Considering that a lot porn (note that there is erotica which is different from porn) is about POWER, submission, fulfilling only the male's desires, where male authority is never challenged by women and women are willing servants, and degrading women rather than just the act of sex, he may find that arousing, though you do say he watches only girl on girl, but a lot of that porn also hints at the male fantasy rather than the act of sex alone. Also, it is an unrealistic fantasy world. That reason alone may be why he does not want to involve you in watching it with him, because he's attempting to escape from reality.

I think it is up to you whether you will be comfortable with a man you are married to to watch porn (your boundaries are important); not all men watch porn, I have male friends who do not watch it and find it disgusting and are not repressed anti-sexual subhumans as some may suggest, if that is part of your worries about whether you should end the marriage or not over the subject of porn. And if he's lying about it, there may be a bigger problem with him than just him watching porn, it could possibly be an addiction.