When you finally said 'I love you' to Sanemi, only one thing crosses his mind once you say those words, "Shit." Why such colorful language you may wonder, though it's because you've caught him off guard my dear. His sword lying loosely at his side, eyes staring holes into your own. As though he was searching for some sort of lie in your own words.
Considers this an indirect devotion. This is sealing your bond, your relationship. In a way, it's as if declaring, "I'm yours!" That he can now let free any foul emotion boiling in his chest. All the jealousy, no longer hiding his lovesick gazes. Simply giving into his pesky feelings, letting his guarded act fall to his feet.
Finally succumbs to his heart and murmurs, "Tch, I love you too." Resting his hand on your head, avoiding your gaze. But I'm sure the drumming of his heart will reach your ears, revealing how much he truly does. The gods know he won't allow anyone else to hear you say such a thing.