I had a geek friend who wouldn't eat anything other than burgers (no cheese) and fries and Coke. (And pancakes or waffles for breakfast.) Then he got married, and because he was actually an adult at that point, he willingly agreed to at least try anything his wife cooked for him.

...it took them six months to figure out he was lactose intolerant. And intolerant to rice, of all things. And soy. And about half-a-dozen other common ingredients in grown-up meals.

After that I decided not to judge anyone who chooses a seriously restricted diet, even if they've never actually gotten diagnosed, because they may have learned from bitter experience that trying new things means misery. (But you know what? The result of that with my friend wasn't going back to nothing but burgers - it was the two of them working together to figure out what he could eat and find interesting recipes that worked...)