In my opinion, the reason why kotaro thought about kyujo while having phone sex with ayumi was because he subconsciously felt guilty that he got kissed by a guy other than ayumi. See at the end of the chapter, when he asked for a kiss from ayumi, he placed it on his forehead where he got kissed by kyujo.
In my opinion, the reason why kotaro thought about kyujo while having phone sex with ayumi was because he subconsciously felt guilty that he got kissed by a guy other than ayumi. See at the end of the chapter, when he asked for a kiss from ayumi, he placed it on his forehead where he got kissed by kyujo.