I can't speak for your boyfriend but I can speak for myself as well as many people in this group. A stutter is often the greatest source of self doubt, anxiety, and sadness for anybody that experiences it. Think of it like having a ball and chain tied to your legs at a track meet but only you can see it. You'll lose everytime even though outwardly you look completely normal. Then imagine you can never ever take it off, constantly wondering why everybody else can move freely while you can't. This (as of right now) is an uncurable disability that has lead many to deep states of depression, and for some, even suicide. I don't say this to scold you. I say it because I want you to understand how much this affects the millions of people that suffer from it. Regardless of how outwardly strong your boyfriend appears, he still struggles daily with situations that you take for granted. Have a serious conversation with him about how calling his stutter "sexy" makes him feel. If I had to guess, it makes him feel like you don't understand his lifelong struggles at all. Every person deals with this differently so a sincere talk is my best recommendation.