What processes regulate the sex of offspring?
In theory spermatozoa are indeed produced in a 50:50 X:Y ratio. However, there are factors which can change the ratio, such as hormone-disrupting agents or even food.

Best known factor in the female body is timing. There are different theories of which The Shettles method is the most well known I think. In this theory they think that boy sperm are faster than girl sperm, but don't live longer. So to have sex around the date of ovulation highers the chance of getting a boy (their sperm is faster), but for a girl sex should be planned a few days before ovulation (they live longer). It is all theory, of course, so there are no guaranties.