@Nova_aren probably for first things, she actually likes the guy. Second, she needs an incredible amount of energy from what is surmised from the first couple chapters. She says her body gets tired and her body stiffens up, so nobody is able to provide her with enough energy to even do lewd things. However, Yuuki has an insane amount of energy, the doc said that he'd have to pair with a succubus queen level of power to have someone who can handle the energy he puts out. So it seems like Nono can do it with Yuuki due to him having insane amount of energy. Unfortunately Yuuki probably thinks it's not ok to do so, and is also being a superb shy boy like most MC's of any romance mangas.