The reason it's rational is this: were GMOs to be found to be unhealthy, it would be a continuation of a well-established pattern. Nearly all food "innovations" of the last 100 years are unhealthy.
- Trans-fats (hydrogenated oils) are basically artery plaque in the form of fake butter or shortening.
- Nearly all additives such as MSG and artificial sweeteners are either unhealthy or suspect.
- Bleaching and other treatments to improve shelf life denude food of its nutrients. Re-enrichment only replaces macronutrients, leaving micronutrients and minerals unaccounted for.
- Preservatives enhance shelf life and reduce the likelihood of food-borne illness, but many are themselves suspect and many have been taken off the market.
- Nitrites are possible carcinogen.
- Google: brominated oil, arsenic poultry.
... and so on. The only beneficial modification to the food system from a nutritional perspective has been fortification with certain key nutrients, namely iodine and vitamins C and D. But we're talking two successes out of dozens of failed experiments.
In every case, the "experts" told people it was safe or even good for you before the experts proceeded to change their minds. Margarine in particular is illustrative. It was recommended as a health food.
As a result, people have become rightly ultra-conservative about food, viewing any modification to food as guilty until proven innocent. "GMO is safe" means "we haven't figured out how GMO kills you, yet."