Inside the Einzbern castle Kiritsugu and Irisviel summon their servant: the most powerful class: Saber, Artoria Pendragon.
To the confusion of the couple, the legendary king of kings was a girl!
Irisviel wanted to be safe and touched Artorias body, stripped her and proudly confirmed with her husband: "Yes, she is indeed a girl Kiritsugu!"

Shortly after the first mana transmission for Artoria began. The best way for a female servant to gain mana is from the semen of her master.
At first Irisviel only helped out, but soon joined the fun.