You don't really have any idea where opposition to GMO foods originates. You guess that it is reflexive and unthinking because that is convenient for you to guess.
There are strong arguments against, not necessarily GMO as a technology, but practically all existing commercial uses of it. Furthermore, each use of it is potentially an independent problem with its own dangers. Thus far the only beneficial use of it I have heard of is "yellow rice", which is not actually available, for purely capitalistic reasons.
They're all beneficial or they wouldn't have gone to this stage.
Maybe you don't think the benefits outweigh the risks, and that's a valid question.
However, the concept obviously has tremendous potential. All the variety of life is after all genetics. Improvements in nitrogen uptake could save tremendous amounts of fertilizer and help the environment, for example.
Everything is beneficial to somebody. Opium distibution is beneficial to poppy farmers, CIA agents, and, earlier, to the British Empire. Roundup-resistant soybeans are beneficial to Bayer.
golden rice is not available because activists in courts have done everything they can to keep it unavailable. This isn't capitalism, other than capitalism will not throw good money after bad fighting things out in court.
There are strong arguments against, not necessarily GMO as a technology, but practically all existing commercial uses of it. Furthermore, each use of it is potentially an independent problem with its own dangers. Thus far the only beneficial use of it I have heard of is "yellow rice", which is not actually available, for purely capitalistic reasons.