There are different rates of conception versus birth. Either females are more likely to be conceived, but males are more likely to survive to birth, or vice-versa. The pH of the vagina has a lot to do with the survival of sperm, and a harsher environment leads to more of the stronger sperm sex (whichever one it is) surviving and thus more likely to impregnate.
The odds though are more 50/50 when looking at the entire population, I doubt it would be as accurate if looking at a small subset of related people. If the vaginal secretions of the womenfolk have a tendency to be more acidic than the population as a whole, then those women would be more likely to give birth to either male or female babies, whichever is strongest.
I'm not sure of the specifics, it's been a while and I'm not going to bother looking up which one is more likely to survive a harsh vagina.