I Can’T Believe You’Re Hard! The Fierce Gay Man And The Virgin With Low Self-Esteem summary:

It's because he sucks and squeezes my boobs...no, no, no! Yuki, who has zero experience with men, falls in love for the first time in her life with her senior at work, Kashiwagi. One day, Yuki happens to witness the scene of Kashiwagi's love affair. What's more, the other guy is a man! No way, Kashiwagi-senpai is a onee-san! Yuki, who still can't give up her feelings for Kashiwagi, offers Kashiwagi a certain condition, and they go beyond "love" and have a very sexual relationship...