Honestly, there are only two reasons to oppose GMO seeds, but both of those reasons are terrifying.
The less-bad of the two is patents. Food should not be patentable.
The other, far more terrifying idea, is
GURT - Genetic Use Restriction Technology. Basically, mega-corps trying to make it so you can't regrow your own crops, but instead have to come back to them every year to buy more seed.