I have a friend who is what you could call asexual. He never showed any interest in women (or men); originally we thought he was just an extremely shy tech-nerd type (he grew up mostly in the company of computers and very few friends), but eventually we discovered he didn't use any porn either.
Later I learned that when he was very small he had a severe nose infection which made him lose his sense of smell completely, and I believe it is very much related to his lack of interest in sex. I guess not being able to sense any scent just blocks your libido. Of course this could be just a happy coincidence but I do think there is a correlation.
I usually find people who claim to be asexual genuinely aren’t and there's other things going on for his/her lack of apparent sexdrive, or they're just lying about it. And ow do you know he doesn't watch porn? I think a few of my mates have denied in the past that they don't watch porn, but I know that they're liars
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I know because I shared a flat with him for a year and all the rest of us would talk openly about porn, girls, etc and he wasn't really shy about it, just uninterested. I doubt very much he would have made such an effort to fool everyone. I also know other people who denied watching porn, but they did have an interest in women/sex so it was quite obvious they lied (or in a minority of cases maybe they really didn't for religious or similar reasons).