I can relate to this, and there are many factors involved...
About 6 years ago, my wife lost all interest in sex... It would happen maaaybe once or twice a month and ONLY when she was in the mood, my mood be damned... I would take what I could get, when I could get it, even if I wasn't in the mood..
Eventually, I gave up asking and trying in general..
About a year and a half ago we went through a REALLY rough patch, with the lack of sex only being a small part of it.. Well, suddenly, realizing she may lose me, she woke up and started over compensating... Offering it up all the time and going overboard on the physical affection... The problem? Since I had pretty much given up on ever having a "normal" sex life, my desire took a nose dive.. It's evened out a bit since then and we go at it usually a couple times a week at this point.. But the problem still remains.. I have no drive, no desire, for her, porn or otherwise...
I have to really work myself up to it, and if something distracts me at all (random thoughts, dogs moving around, etc) I lose it, leaving both of us disappointed..
Other factors like age (41) and weight (heavy but not outrageously so) really make a difference, I've found. I've tried medication and it works a little, but not near as well as I'd like.. It does nothing for the lack of actual desire...