Where is the source that to be mekayem Pirya Vrivya one must have at least one boy and one girl? Does that mean that all the many families I know that have all boys or girls have not fulfilled their mitzvah?
The source of the requirement to beget a son and a daughter is the ruling of Beis Hillel in the Mishnah (Yevamos 62a), according to which one does not fulfill the mitzvah without at least one son and one daughter. This is the opinion that is ruled by Rambam (chap. 15 of Ishus) and all poskim (see Shulchan Aruch, Even Ha’ezer 1), and yes, families that have all boys or all girls have not fulfilled the mitzvah. However, even somebody who doesn’t have a boy and a girl can fulfill the mitzvah by virtue of his grandchildren (Rambam, Ishus 15:5).