Natural selection and GMO modification that is done today are very different in principle. Imagine DNA as an enormously long and complicated string of numbers. This is a simplified version that contains only basic structure of a tomato, without representing the actual length and complication of each segment:
Every single abbreviation there contains a very complex structure that varies in length, up to 100000 base pairs in length. The problem is, when you try and insert moth DNA block into corn (to reduce crop damage from its caterpillars), the latter has to change in size, which in turn shifts all other parameters. It is an incredibly complex procedure to make. Right now, we are at a beginning stage of the development of that tech. We can shift certain blocks, replace them, but it is not as some would describe an exact science in a classical sense. There are no scissors to cut a DNA pair and then stitch it together. It is all done using different acids and chemicals to dissolve certain parts. Each batch we create is a little bit different. Even things that are put to mass production, like corn seeds, can be distinguished from each other according to batch number by testing their DNA. Because of that incredible complexity and the fact that we are able to modify DNA only to approximation, risks of creating new traits that were not intended are extremely high. This is not just theory. We have already seen some cases when a genetically engineered plant cross-contaminates a nearby specie and completely destroys its population for all intents and purposes (makes F1 yield sterile). A possibility of a new, unexpected chemical produced in a system that would have a dual purpose of not only rupturing caterpillar organs, but actually damaging more complex organisms is far from zero. That is the main reason I would personally stick to natural selection as opposed to direct gene manipulation.
Another reason is that GMOs are over advertised and do not particularly solve food problems. I know that some people claim that if we convert everything to a GMO, we would increase our crop yields to feed everyone and still have some left. But that is simply not true. According to UCS, GMO yields grew only by 3.5% between 1996 and 2009. That can be easily attributed to improvement in farming tech. Fact is, there is no difference in yields between GMO and non GMO crops. NY Times even made an article about it a while back.
https://www.nytimes.com/interactive/...esticides.html
But naturally, it was largely ignored. I am not saying that GMOs are nukes in your field or anything like that. But everyone should be given a possibility to chose for themselves. I do not want to risk it for a marginal 1% better yield or price reduction.