The regulation of GMO foods is a pretty hot topic right now. Europe requires GMO foods to be marked and there are states, such as California, who are considering similar legislation.
Most people who are pushing these regulations argue that moving genes from one organism to another is "unnatural" and potentially dangerous. The problem with that stance is that it is completely at odds with the natural world. We can watch the movement of genes between unrelated prokaryotes in real time (think, the spread of antibiotic resistance) but this process is not, as often believed, relegated to the ranks of the non-eukaryotic world.
Among the many things the modern genomic age has taught us, it has become unambiguously clear that genes move between unrelated eukaryotes. Examples abound, but a review of the Choanoflagellate (Monosiga brevicollis) genome (Tucker 2012) shows that there are roughly 1000 genes of foreign affiliations encoded there. Choanoflagellates are the sister group to Metazoans (animals), so they are not some obscure and distant branch of life (unless you only think about rats or flies, then maybe they are). And much like other organisms that have been examined, the foreign genes come from a wide range of organisms, including vertebrates, plants, algae and bacteria.
And whereas the presentation of these data is a bit dubious at times(how often do you see "phylogenetic trees" with no support values and no indication of what kind of tree they are or what models were used?) the fact is that this is not a unique case. EVERY genome tells a similar story, to varying degrees.
So if you want to make a case against GMOs, fine. Just don't use the "unnatural" gene transfer angle as the main thrust of your argument. It just shows you have no appreciation for how much nature likes to mess with things.
Tucker RP (2012). Horizontal Gene Transfer in Choanoflagellates. Journal of experimental zoology. Part B, Molecular and developmental evolution, 1-9 PMID: 22997182