Maybe I'm reading too much into it, however, let me try and play psychologist for a bit:
We know Yokoi is a loner, who has trouble dealing with people and didn'/doesn't have any close friends (or friends at all), but isn't socially inept and does have the ability to develop feelings for the opposite sex. He's also very introspective, with a tendency to look at the unusual side of things, the one most people try to avoid, the darkness everyone knows of, but are too scared to face. One of Yokoi's stories mentioned the unforgiving nature of a sea turtle's life, which leads me to believe he is (or used to be) interested in animal documentaries. On one such watching, at a younger age, he could have learned about how dogs smell each other's butts to know all sorts of things about each other. Having no peers to speak to or hold back his weird thoughts, he could have then pondered why humans don't do so, and if it was actually possible to understand something about a person from smelling their farts. At that point, though Yokoi claims to have an aversion to people, I'd like to believe he, like most of us, had a subconscious desire for someone he could be intimate with. So intimate, in fact, that even sleeping next to each other and smelling the other's farts (a somewhat common occurence when you've spent a long time together with a lover, for example) would be commonplace. Coupled with teenage hormones intensifying his emotions at the time, that initial curiosity about intestinal gases might have been elevated to fetish status. Finally, smelling a girl's fart from a bag might be from wanting to have a more "concentrated" experience than just catching the scent in the air.
Thanks for coming to my TED talk. Now, if you'll excuse me, I need to take a shower. Or several.