what...

Because as the seme says, the uke has a weird idea of fidelity. The uke thinks that as long as he's loyal to the seme in affection-- as in, his heart doesn't wander over to others-- he's not cheating on the seme. After all, to him, he's only having sex with others to take care of his needs, so it's not considered cheating. For instance, we hug our friends if we feel lonely, but we don't consider that cheating on our partners. That's the type of mentality the uke has, which is why he says, "But he's a friend (not a stranger)!"

Of course, I don't agree with this at all... I mean, I can see that the uke didn't cheat with malicious intent, but still... he's an older lover, right? He should know by now what a relationship entails, and what to do and what not to do...

But I feel like the last page is showing us that he doesn't (he goes "Okay, if you're sad I won't cheat anymore"), as if he still doesn't really understand why he's hurting the seme and will simply stop it to satisfy him.

So I guess it's more an issue of bad characterization (uke doesn't know shit about society?) than a character flaw? Which is why the seme is willing to accept him?