So, two things. First, this work has two timelines, the "kiss" timeline we see here and the "sex" timeline where he has to engage in coitus to survive. If you are eager to see our friend bang, including his ex, you can get some of that at the source.
Second, casual sex is not the consequences-free activity so many of you think it is. There are psychological and physiological consequences that should be considered. Now, is our boy Kazuya thinking of those? No, but I respect him for sticking to his principles over thoughlessly sticking his penis into the first hole on offer.