@jokerxhisoka
Because they both still have hang-ups beaten into them for all of their lives until now.
You have Nono, who is seen as a failure of a succubus, and is pretty well known amongst other succubi for that, and that's done a number on her self-confidence. Then there's our lad Yuuki who has so much vitality that he'd literally kill any woman he slept with, even succubi (he can even resist their Charm, though Nono's own Charm caught him briefly), and like Nono, this is known by those in his general area and he has faced a level of ostracism.
These things cannot be immediately solved.