From Hoshikuzuu Scans:

Kiyomizu, under the impression that he was going to a drinking party with colleagues, is tricked by Karasuma into a mixer with feisty cougars. Sensing that he’s going to be devoured by them, he devises a plan and desperate tells them that he is gay, and then continues to give Karasuma a kiss to prove that he wasn’t joking. After leaving the store, the girls are undeterred and to drive his point home, he goes to a love hotel!

Alcohol coupled with being in a love hotel prompts Karasuma to say “Let’s try doing it! Just the tip!” His curiosity was just too strong. He was willing to be on the bottom and Kiyomizu reluctantly agrees. In the beginning, Karasuma is complaining that it doesn’t feel good, but it slowly becomes pleasurable as he lets out the cutest moans, drawing Kiyomizu in…!