> the expectation for offspring is equal between male and female so the selection pressure generally keeps it close to 50
It's the other way around: the selection pressure drives the expected offspring to become equal. The expected offspring for an individual of a certain gender is (total offspring)/(number of individuals of that gender). The fewer individuals of one gender there are, the higher the expected offspring and the greater the selective pressure to produce more offspring of that gender. So the gender ratio is self-stabilizing.
It's the other way around: the selection pressure drives the expected offspring to become equal. The expected offspring for an individual of a certain gender is (total offspring)/(number of individuals of that gender). The fewer individuals of one gender there are, the higher the expected offspring and the greater the selective pressure to produce more offspring of that gender. So the gender ratio is self-stabilizing.