There's so many biological inaccuracies here it's staggering. Couldn't they have made them Parthenogenic? I know they needed a reason for the Dragons to be defeated, but it would make SO much more sense.
And been a more precient threat--"these things don't even need to mate like humans do. Whenever conditions are optimal, they...not REproduce; just PRODUCE young. If we don't find a way to wipe them out....they'll just crowd humanity out and into extinction."
[Not scientifically accurate description, but close enough in a Hollywood/Dramatic sense. ]