Your post made me realize that in some ways, Kosuda was asking himself wether he was with Yamada just for sex or not... Which is Yamada's objective as a front at least. Quite funny if you ask me.
His reaction is very normal for a 15 years old guy, full or hormones, that would go anyways to get his first time... or not! Reminds me of these wonderful days when sex was in like 99% of my thoughts.