In the middle ages, people believed that sperm coming from the left testicle produced girls. Men who wanted sons had it removed sometimes.
In the Middle Ages, men who wanted a boy sometimes had their left testicle removed. This was because people believed that the right testicle made "boy" sperm and the left made "girl" sperm. None of the various high and low-tech methods of today that claim to be able to influence whether you'll have a boy or a girl are quite so drastic. Since you want a girl, a method called MicroSort is available that can increase your chances of having a girl to around 90%. Of course, it isn't free.