Inma Hiroimashita. - Arasaa Shojo ga Inma no Ban ni Sareru made

A Sexually Frustrated, Virginal Spinster and the Hungry Incubus she picked up. The weird animal I found was actually an incubus!? Time spent without a boyfriend = whole life, our spinster protagonist gets swept away by an icubus' relentless attacks and ends up having her first time.. But a virgin committing that "act" with an incubus is actually grounds for a ritual.. It's a book about having musclebound sex with a incubus that has a completely different physique.