not sure if any parent sex checks are involved in pregnancy at all yet. two versions ago it was explicitly off, so even males could get pregnant.
the way it works now is anytime someone has an orgasm during a sex act with a partner, the partner has a 35% chance to get pregnant.
this includes situations like female-female facesitting.
but again, i do not think it's really a bug, only that sensible constraints have not yet been added to the system.