I was wondering, is there some algorithm that tries to balance boys and girls? Or is the sex decided by 'random' means? I ask because in my last game I had a decided majority of males for much of the time, whereas in the current game the majority are female. The last game balanced out by the time I got to 90 villagers; I'm only on 45 at the moment with the present game so can I expect that to balance out too?

From all I have read from LDW staff, it is strictly a random event as to if you have a boy or girl. And like any "random" thing you will have boom's of girls or boys, but as you said it will average out over time.

Yes, it's almost averaged out. I've reached 90 villagers now, (no babies) and have 49 females and 41 males.