The presumption is that GM crops are superior. Issues like world hunger can be conquered. Crops that withstand drought and pests can produce higher yeilds. Nutritional values and shelf life are increased. So what's the big deal?
The more GM crops are grown, they more they become a part of our food chain. Harmful chemicals become part of a regular diet through the consumption of fruits and vegetables, or animals that have eaten GM crops.
Since labeling GMO food is voluntary, the only way to avoid them is consume only organic foods. If a product is certified organic, it cannot have GMO ingredients. Some people argue that organic food is more expensive, but the health risks from GMO food are becoming more well known. Would you rather spend your hard-earned money on sustenance that edifies your body, or medicine to fix it?
References:
Hever, J. (2011). Controversy Clarified. The Complete Idiot's Guide to Plant-Based Nutrition (pp. 95-96). New York, NY: Penguin Group (USA) Inc.