My older brother had this affliction. My kids have this affliction. I don't know why they all just can't get along but they hate it when their food is touching another food. This is a classic case of dintercourse and from a kid's point of view food mixing flavors is unholy.
I am not sure where they get this notion because they do like to mix Fruity Pebbles with just about anything. When I suggested that at dinnertime we make a Taco Salad instead of just having separate sections of food items they looked at me like I had a second head.
I am trying to teach them that sometimes the best recipes are created from combining two or more flavors together but they still seem skeptical. So, I tried to explain it in a language they would understand like DESSERT.
I asked them if they liked marshmallows, graham crackers, and chocolate by itself and then demonstrated the flavor explosion that a S'more has when combined. They were on board with this notion finally and then I tried to sell them guacamole.
The thing is, they love onions, tomatoes, cilantro, and lemons/lime. But the instance that the avocado joined the party all bets were off. Maybe it is the bright green color that lost them but my son wouldn't even try it. Oh well. I tried. Lucky for my I LOVE dintercourse so my guacamole is safe.