I'm new here so I can't leave a comment but part of your biological assumptions are wrong. In humans, the baby's gender is controlled by the male's contribution, ie whether they give an XX or XY gamete. If the Y chromosome is present, then the baby becomes male. In its absence, female. So the "default," in a way, is female.
I see know the questioner addressed this in his post, but stick with me.
I find it unlikely a woman could biologically assert a choice of her baby's gender. Would she have to constantly focus on the baby's gender to ensure the right hormones are released? Or just once, at some turn point in gestation, like week 5? What happens if she changes her mind, or just honestly never makes a choice? Because of the passivity inherent in pregnancy, I don't see her evolving any sort of conscious hormonal control. 429
I can't think of any examples of animals consciously controlling hormonal release, for very good reasons.
Additionally, the actual act of procreation for women, is again, passive. So if there was to be a choice, it would have to be a choice made during an actual conscious action. For this, I would choose ejaculation. As I postulated in the comments, a man's testes could each produce male- or female-begetting gametes only. A muscle that closes only one vas deferens and allows the man to choose female or male semen seems to me far more biologically plausible than the woman.
One way I feel a woman could plausibly effect the sex of her baby is: Perhaps a male embryo releases a hormone that causes an itching sensation, and she could self-abort.
So...
No matter who picks, a child that knows his/her gender was chosen by one parent is going to feel very beholden to that parent, especially their expectations. A son would feel very strongly that he has to live up to his father's expectations. In a less-progressive society, this would expand gender roles.
Similarly, a daughter would feel the same pressures, but the opposite way: virginity would be more highly valued, and she would respect her father's vision for her as a girl.
Of course, broad strokes here, but someone knowing their gender was selected for a reason would face enormous pressure to live up to the expectations of that gender. 1008