Uke cant be aroused with sex. Seme finds out what place can make uke aroused. (In the mouth!!)
Seme approached uke in pretense of teaching him math even tho seme is good at it. Uke knew at one point seme is good at math but still taught him, so when seme confessed to uke, he agreed and they started to date.
Kuchi no Naka no Shita