Most non-GMO hybrid plants do this. It is not anything specific to GMOs. The only plants guaranteed to produce viable seeds are heirloom plants, which are usually less hardy and resistant to disease. (My two favorite tomato varieties, Brandywine and Mr. Stripey are both heirloom.)
There is still a huge market for heirloom plants, btw. So it's not like anyone is forced to buy from Monsanto; there are other varieties of plants available.
Seed Savers Exchange is a repository of heirloom plant varieties available to anyone. I keep meaning to join them.
He may be referring to terminator seeds, which were something investigated by Monsanto (and other biotech firms) but never commercialized. Let me repeat that: terminator seeds are and were never sold to the public.
I really think the parallel to global warming skeptics is very apt here. Please don't take this as an offense if you are a particularly liberal person; I am myself quite liberal. I think it may offer a chance to understand those who you don't normally agree with, and give you some empathy.
I've had lengthy (and I mean
lengthy) conversations with global warming skeptics, and they offer the same sort of evidence; all vague, unsubstantiated claims of corruption amongst climate scientists and fraud by the government. When you systematically explain why these claims are false or wrong, they often resort back to "Well, I just don't trust the government to handle things like this, and I think regulation can be burdensome and cause more problems than it solves."
Just as many conservatives have a gut distrust of government intervention, many liberals exhibit this same sort of gut distrust of large corporations. I think this "gut feeling" can sometimes override sense, and produce vague, wishy-washy hatred for something that isn't really founded on good reason.
I'm not trying to convince you to abandon liberalism here -- far from it. I'm only trying to show a thought process that everyone falls prey to at some point; our higher order beliefs are often based fundamentally on our "gut feelings." You strip away bad arguments and find, at its base, that the person just doesn't like a huge company patenting food, or just doesn't like the government regulating free trade, or just doesn't like being forced to inject themselves with vaccines, and so forth. The gut feeling leads, and then we form arguments around that gut feeling.