First of all, "Like to have sex" is not synonym for cheating. Now, the one who should read carefully is you it seems. From the manga it states that it's three years into their relationship. The seme was the one who wanted the serious relationship from the start and deleted all the uke's contacts from his phone. The seme never spares a thought after the uke. He goes after the culprit even after the uke asked him to stay.
They weren't serious from the start, but then the seme is the one that told the uke to have a serious relationship with him, but when they do, the seme cheats on him many time...
Yeah, sorry I worded it wrong. The seme was the one who suggested they start going out seriously first
no you are right and I was replying to the main post :3
If you read carefully both the Seme and Uke at the last story like to have sex. they didn't considered their relationship serious, for them it was for fun... The Uke realized he doesn't want to share his man to anyone anymore. And when the seme saw the uke being push he realized he can't live without the Uke. He just run to culprit to solve the problem. And he thinks their bestfriend is there to take care of the uke while he was solvinf the issue.
at the end they realized they love each other and won't cheat anymore....