If the sex is prolonged and she experiences orgasm, then the chemistry in her body will be prone to male to develop. If the sex is short and if she does not reach orgasm, then her chemistry will be prone to have a female.

From my years in Asia...

Another factor to the determination is how far they have to swim.  I am not sure which is which but one sex is quicker but tires fast, a sprinter, and one is more of a marathoner.  Depending on where in the fallopian tube the egg is varies the probability of one sex over the other.  The temperature tracking to try to determine when ovulation occurs and then either try early or wait a little longer.  

There are probably many different factors, chemical composition, force of ejaculation, anatomy of the woman and just plain luck. Someone has to be first and from there on everyone else just does not count. Winner take all.