Felix indeed does not know just how sexy Byleth finds him, mostly because he's never in his life gone out of his way to try to be sexy. Not yet, anyway. It's not impossible that he might someday, though...

The way Byleth moans and pushes a little further in, the thick desire in his voice, the way he pulls on the leash as if to say you're mine or you're not going anywhere because I want you right here - it all makes Felix flushed with arousal, on top of the heady feeling of Byleth slowly filling him again. The muscles in his thighs flex as he lowers himself little by little; his head tips forward as he tries to steady his heavy breathing, the tangled curtain of his dark hair falling over his face. He presses both hands against the professor's chest to help keep his balance.