Don't be so hasty!
Looking close to home, we find that our own bodies contain many foreign genes. It is estimated that around 8% of the human genome consists of fragments of endogenous retroviruses - about 100,000 of them. Not all of these fragments are "junk" (a term the popular press is rather keen on). A number of viral genes have been co-opted for our own purposes, in gene regulation, production of transfer RNA and ribosomal RNA. One viral gene is critical to the formation of the placenta.
On this basis, I'm not too surprised to read a piece of research* that shows that some of our vegetable crops are naturally transgenic. Cultivated sweet potatoes contain the transfer DNA sequences from a bacterium called Agrobacterium. This genus naturally infects the roots of certain plants, causing a nodule or hairy roots. This T-DNA is not present in the wild type sweet potatoes, implying that one or more traits carried on this piece of DNA were selected for during the domestication of the sweet potato.
The authors of the paper point out that sweet potatoes have been consumed for millennia, and that this "may change the paradigm governing the “unnatural” status of transgenic crops."
If we look further, in my opinion, it is almost certain that we will find other bacterial or viral genes in our fruits and vegetables.
* The article is technical, but you can find it online
The genome of cultivated sweet potato contains Agrobacterium T-DNAs with expressed genes: An example of a naturally transgenic food crop