why would it be illegal to similarly choose your ride driver by gender?
Because it’s against the law, as it is written. It isn’t a BFOQ for a taxi driver to be male, female, young, old, of any particular race or religion, so yeah, discrimination on those qualities clearly violated the law.
Wouldn’t discrimination be more if Lyft refused to hire male drivers or something to that effect?
Preferentially encouraging discrimination against male drivers is still discrimination, even if male drivers are still allowed on the platform.