Basically, the main girl of C&V, Risa, was someone Akutsu was interested in.
The main guy of C&V, Fukami, was someone Natsu was interested in.
Natsu and Risa are best friends from before the story even starts.
Since main couple in C&V are canon and in love with each other, Akutsu and Natsu met and ended up becoming sex friends very quickly.
Although this happened, she wants what Risa and Fukami have.
Akutsu is a big CEO and sleeps with many women. However, when he meets Natsu, he genuinely falls for her.