The uke thinks it's too good to be true so he's testing the seme's feelings for him by bringing up cheating. He's trying to gage how far he can take to see if he'll stay and won't leave him. Unfortunately, there are people like that in the real world. If they keep pushing to see if their lover really loves them, they're gonna push them out the door.
That's exactly the plot of "Overdose", but to be honest I dont really think is the case in this manga, he is not trying to prove his lover devotion but trying to make sure about his feelings for Kayano, he has always just gone along with people who likes him, so he is not sure if he is just going with the flow or the money is makeing him so happy that he thinks he loves him for it, still he is an ass, I don't hate him but he makes it really hard to like him.
Honestly, I was hoping he and his dark haired friend would become a couple. I feel so sad for him. Don't care for the other guy.
haha me too