While many believe genetic modification happens naturally, the use of selective breeding is not comparable to splicing and inserting new genes. This practice uses tissue from animals to alter the DNA of the plant. The results are unknown at this point but the possibility of a mutation is likely and at some point we will have to deal with the effects of this. In my opinion, I find it unethical to cross breed between animals, humans, or plants.
Modifying the genetic code of any living organism (especially humans) might be more hazardous than it is unethical, but I think it being dangerous makes it morally inadvisable. I've seen enough movies (e.G. Jurassic Park, Iron Man 3) to conclude that biological engineering, biotechnology, etc. is not worth meddling with.
It is extremely risky to modify the genetic code of any living thing but the possibilities of what can be done if we can learn how to properly manipulate the genetic code with minimum harm are endless. I guess it could be considered immoral in how we decide to use it.
If a genetic modification, a specific one can be proven to be safe then it's not immoral or unethical. Some people say it's unnatural. So are antibiotics, surgeries, painkillers, the internet,...
Here's an example of genetic modification I think any sane person would agree is ethical:
These kids would have died young if it wasn't for these clinical trials. I hope the trials go well and I hope that soon this therapy for Severe Combined Immunodeficiency Syndrome becomes commonplace.
Genetic engineering is just a tool. It is how it is used that is ethical or unethical.