It has nothing to do with finding bodily functions disgusting, it's just a pavlovian reflex.
Let's say, you are a lonely virgin, and you watch porn videos every night. Just like Pavlov conditioned the dogs to start salivating from a surrounding effect instead of the food itself, you will eventually condition yourself to get a boner not from the female body itself, but from the act of sitting in front of porntube with your dick in your hand, and clicking on videos. There, you are no longer attracted to women, you are attracted to porn.
It's the same with 2d girls. You watch lots of anime, there is one girl that looks kinda attractive, after all, her body does imitate RL women. So you start fantasizing about her. You focus on her or on other anime girls too much, and you will trick your brain into believing that you are only getting turned on by the large eyes and the lack of noses and monocolor complexions, and not just from primary sexual characteristics.
This is how all fetishes are created. There is some circumstantial element appearing in much of your sexual fantasies, like a setting, a clothing, a relationship dynamic, or a pose, and by focusing too much on it, you turn it into a more and more necessary part of you getting turned on.
