Well, he's head over heels for Kabu so maybe he found it hard to lie about that...ooooor he just craves for the punishment entailed with it???(￣∇￣")
I think Nirasawa thought Kabu wouldn't mind because he let one of his men in Chp1 play with his ass (with toys), while they were torturing him. Nirasawa probably thought that compared to that time, it would mean nothing to Kabu if Kouki came on his pants.
Not to mention that Nirasawa always thought that he had to make a lot of money in order to help Kabu, so he probably thought Kabu would be happy that Kouki didn't take any of the money in exchange for "so little."
Nirasawa is cute and naïve, almost like a kid at times. Although he kind of gets that Kabu loves him, I don't believe he yet realizes just how much he means to him because Kabu never says it. I think that's the main reason he didn't realize telling Kabu about Kouki's little indiscretion would lead to him being ravished by Kabu until he passed out.
"I was made to pay with my body." Nirasawa just had to go spill the beans, didn't he? How can someone be so naive