You don't know what you're eating
With GMOs, the food you are eating is not natural and could have some side effects. GMOs can actually trigger specific allergens. It will be hard to stray away from GMOs in the future due to the fact that there is many ways that GMOs can go wrong in the long run. Tampering with nature is unethical and the change of it can change the course of agricultural history. The products made from GMOs should be labeled so that consumers can avoid them if they so choose.
Nature is not meant to be tampered with and there is a very big chance that the course we are heading in will be having only GMOs on the earth. There will be no "fresh" produce and if that happens there will probably never be anymore fresh produce on earth. We have no clue what the side effects in the long run may be as well.