[Disclaimer] This anime is intended for mature audiences, it is not hentai, but it does have its fair share of naked bouncing boobies.
So i finished everything available from High School DXD. In it, Issei more or less gains power from getting turned on.
It got me trying to remember another anime that uses this idea.However, it was much more blunt about the idea of the power of horniness.
The main protagonist finds a Magical Girl standing in the rain. Feeling sorry for her, he invites her to his home so she could dry off. Once she showers off, she forcibly seals a contract with him by stabbing him in the chest with half of a demonic sword. As it turns out, she gains power by taking some form of energy from the living, and she was searching for a certain man who could give her virtually limitless amounts of that energy. She made the contract with our hero so she could use him as a temporary source of energy during her search. He is not the one she was searching for, but he does produce an unusual amount of power. The catch is, that it only really works when he gets turned-on. In battle, she would transform into black "armor" and wield what i think was a scythe. The "armor" in turn would get the main protagonist all hot and bothered, which resulted in him having more energy for the girl to use. It was comedy, action, and romance.
I'd really like to watch it again if i could only think of the title.