Seems like the Seme was in love with another coworker (boss Uke) and he only has a sexual relationship with the Uke due to their compatibility. The Uke falls for him anyways and decided to declare his love and will to make the Seme fall for him. From the comments though I don't think I should read the main story because apparently the Seme from this one SA the boss in the main story
Eishi Tsuji Doesn't Do Love