Warning: there has been no scientific tests or experiments to prove the validity of the below provided means of measurement.Male sperm cells are said to be stronger than female sperm cells. That means that on the epic race to the Egg, they can wade their way faster than the female cells. Strong as they are they, however, have a short life span. That means that if they want to get there first, they have to give it all they have before they expire.
Female sperm cells, though weaker, have longer lives. Sounds a lot like people, right? Men are biologically stronger than women, however, women almost always outlive their male counterparts. The same characteristics are traced back to the sperm cells that determine a person's sex on conception.
Applying such concepts then, if the sperm cells have to swim a longer distance then the males will die and a female will win. However, if the travel distance is short then a strong male sperm that can withstand the acidic embryo of an egg has a higher likelihood of making it to the goal.
In plain terms: if the thing is short then the males would die before reaching the egg, and you probably have daughters; if the thing is long then they'd win the swimming contest with brute strength, and you probably have sons.