The only thing that stands out is the fact he is lying about it! There could be lots of reasons he feels he needs to hide this. Some may have to do with you, other with him!

-Do you ever make him feel guilty or embarassed about it? That would be a good reason to deny it!
-Do you have a tendency to tell your girlfriends, or worse, family about it?
-Maybe he has some fetishes he is a little embarassed of.
-Maybe it is not the porn, but the accompanying masturbation he is embarassed of?

There could be tons of reasons. The point is not the porn, but find out why the gaps in trust and communication exist.

Just my thoughts!

~Mr. Gentlenibbles