Every single baby I've produced in this game (which has to be over a dozen at this point) has been female with the exception of ONE boy that was twin #2 in a set.
The twins just born in my legacy are boy/girl. Otherwise, I've only had boys for generations of three families. It's possible that it's random because probability isn't perfect, but it seems to me that this ought to be a lot more random than what it is, and clearly I'm not the only one who's complaining about the way that it's happening.
On the flip side, running a matriarchy is interesting if the only time you're likely to get girls is when you have multiples. It means that every generation has to have a set of twins or triplets somewhere in it.
I also think that if this was a matter of probability not being all that precise, that it would go both ways. Most of us having problems seem to be having issues with too many boys and not with too many girls (though there are obviously exceptions to this rule as well).
Since this started for me, I've yet to have a family where the ratios are even remotely even. It has consistently been about 3/1 boys. Eventually probability should start to even out. I'm keeping a family tree on the Drake's, so it should be visible over time. Time, therefore, should tell and eventually it should be close to 50/50.