1. The original motivation for opposition to GMO was mainly religious I believe ("Don't mess with the creation"), but somehow turned into "GMO is dangerous".

2. While GMO by itself is not dangerous a lot of the things that are done with it are. Roundup is very evil stuff, and GMO made it possible.

3. Another problem is also the business model behind a lot of commercial GMO crops, where the farmer essentially signs away all their rights.

2 and 3 are good reasons to be opposed to a lot of GMO.