The genetics of sex determination is still a very new field (well, genetics is a new field in general), but i remember it being my most interesting class from human genetics.

The things about males sperm swimming faster , i've heard it too (but never seen any studies to back it up), but like i said above, that would all come into warping the stats so to speak before the egg is fertalized (i.e. more male sperm around the egg than females, hence changing the 50/50 chance).

But by pure theory - same with tossing a coin - if you do it enough times (100, 1000, 5000 etc) you should see it straighten out in the absence of any genetic mutations or envriomental influences...

If you are only talking about families of 3 - 4 kids per generation, its just not enough incidences to see a trend.