If Nasa never shows any interest in girls other than Tsukasa, it would allow girls to feel at ease in his company. At the end of the day, his company would also be useful, seeing how smart and capable he is. Of course there might also be girls who would take his disinterest as a challenge, should they be interested in him. All in all, this isn't really that sort of a story, though. It's mainly the general circumstances that led to his female oriented group of acquaintances. The bathhouse simply happens to have two daughters. The school he's part-time teaching at is a girls-school, isn't it? Or maybe I'm remembering that wrong. Considering Nasa isn't exactly going out of his way to look for new friends (he can't even have school/uni buddies, since he dropped out of education), random circumstances that resulted in acquiring a few friends/acquaintances are all he has got.