I'm not 100% on it either, but I do feel that it can't be pure genetics. I get that genes are complicated and interact with each other and the environment, but. There's no way the human genome is detailed enough to code for "balloons being inflated" or "the cars from Cars" (actual fetish, I'm so sorry I know and I'm so sorry I told you).
The best defense I would offer of the theory is that its all about having your first moment of "what is this feeling?" at a strange time. There are a few common threads among fetishes, like things getting put into things. Example, one poor sap wrote to Dan Savages' sex advice column about the fact that he had a scuba gear fetish and no one else on the internet seemed to, and the reply he got was "that sucks, you probably saw someone attractive putting on scuba gear". Also why feet could be such a common fetish, since people take their feet in and out of shoes/socks so often even in public. Other treads: power exchange (hence most of BDSM), and cartoon characters that are clearly in a different category from humans, but human enough. To paraphrase the creator of Gravity Falls: "when you're going through all the unexpected changes of puberty, anything with confidence can be attractive. Even a triangle."
Edit: I mean, we're talking about anime. Considering the current state of the market, its not an entirely irrelevant conversation. Still a derail, but its like the train was derailed onto another, paralleled track.