All eggs are X; sperm can be X or Y.
Keep in mind that these people live in, at best, Medieval cultures. In Medieval times, it was believed that the woman determined everything about the baby.
The sperm from the father is a mix of X(female) and Y(male). Conditions in the vagina have a lot to do with which type is more likely to get to the egg. So the mom really does more to determine the sex of the child than the dad.
From wiki answers:
- If the X sperm unite with female egg then a female child is born.
- If Y sperm unite with female egg then a male child is produced.
- As same number of Y and X sperms are deposited in the vagina; which type of sperm ultimately will unite with female egg depends on the conditions of female reproductive system. Whether the condition is favorable for the rapid and safe movement of X sperms or the Y sperms.
- The Y sperms are very delicate and are easily get destroyed by acidic and toxic environment of vagina. Thus if the vagina is very acidic most of the Y sperms are destroyed the rest may get immobile and not fit to swim in the cervical canal.
- The X sperms are stronger and can withstand the acidity and toxicity better.
- Thus if the medium of the reproductive organ mainly vagina, cervix, uterus and fallopian tubes is not favorable then the Y sperms get destroyed or immobile and the surviving X sperms swim in to unite with X egg of the woman to produce female child. This is why failure of family planning method like condom, spermicides, jelly, or withdrawal mostly results in female child. Similarly women having vaginitis, cervicitis, leucorhea, candida infection, cervical erosion almost always produce female children.