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.