After the gentle handling of Marianne, more students seemed more eager to volunteer but suddenly a voice spoke up and much to the girls' surprise, it was Flayn.

Despite having been forbidden by Seteth to attend such classes, Flayn snuck in after overhearing Lysithea and Edelgard speak of their experiences and now she wants to experience for herself how it is to copulate be it for pleasure or for furthering family lines. Despite Byleth's initial refusal since Flayn is not officially a student of any House, he acquiesced with her persistence.

Source: https://www.pixiv.net/en/artworks/147283248