Almost all of the genetic engineering done by Monsanto involves genes that control the expressivity of existing genes, that is to say no new proteins are created.
When foreign genes are inserted into an organism, they come from existing organisms (example, a gene from potatoes is inserted into maize) and don't provide anything new. It wouldn't make much sense to say one is willing to eat corn and potatoes, but not corn with potato aspects. (Oversimplification and gene cartelization notwithstanding if you'll pardon my pejorative indulgences.)
It's not like they're inventing totally new sequences of exons of dubious chemical activity and putting them into products. The actual process is very safe and quite unexciting compared to what they did if they actually inserted real designer genes into the crops.
Furthermore, it makes perfect biological sense to modify your seeds so the resulting organisms cannot breed; one would be introducing an entirely new genome into the ecosystem with unknown and potentially catastrophic results were it able to reproduce and mutate in the wild unchecked. This policy enables Monsanto to maintain a relatively constant and static genome with consistent quality in the end product across the spectrum, while allaying the reasonable concerns of environmentalists and the unreasonable fears of those who simply haven't yet obtained a sufficient education in molecular biology.
We do have the ability to do precise gene replication, at least as precise as normal reproduction allows; that's what a polymerase chain reaction does. Sequencing is another matter, and the speed and precision thereof goes hand in hand with the cost of the equipment and methods, of which Monsanto certainly has the financing and client base to utilize.
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