It isn't entirely uncommon in myth and legend for a woman to become pregnant without the participation of a (human) man. It's rather less common to find stories in which pregnancy is attributed to sexual activity between women--whether, as in this case, with divine assistance, or as in the case in an early Irish text, where the sperm is leftover from one of the women's prior heterosexual activity. With all the fertility technology we have today, the idea of two women being co-genetic parents of a child is still mostly theoretical.