Our food always consisted of GMOs, only the method of modification is not just selective breeding anymore, but also genetic engineering. The arguments against GMOs as human food are, that results/side effects of the manipulation are not extensively researched, that biodiversity will be reduced, that the impact on other lifeforms in the same biosphere will be destabilizing, that the dependency on specific products and companies will be favoured, and many more. The arguments for GMOs as human food are about productivity. The UN estimates that about 805 million of the 7.3 billion people in the world were suffering from chronic undernourishment in 2012-2014. If the same agricultural area yields larger crops, it fights both world hunger and the need for deforestation. Personally, I think that genetic modification will be necessary to provide enough food for the increasing world population, but worry that they create proprietary means in the hands of the few patent-holders.
Disclaimer : Picture is not an accurate depiction of the average GMO tomato.