All right, so the horrible memory that I have, I've gone and forgotten the title of this Yaoi, which I may in fact have on my laptop as it is, but seeing as there's enough manga on my laptop to rival the number uploaded onto manga hosting sites. Going through the entire list... Not a pretty picture. Basically, it'a about a couple currently going out, living together in fact, and the seme has a tendency to cheat, because apparently, the sex is so much better after he's slept with someone he doesn't love. And he's also using it as a way to confirm of the uke's feelings for him.

They've a common friend who's a doctor, and the uke goes crying to him most of the time. Finally, the uke gets so fed up and says he's going to cheat on the seme. He meets an ex, gets treated to dinner, said ex then gives the uke his scarf. Seme manages to find uke, but uke gets pushed down a flight of stairs, and the doctor catches the uke instead of the seme. The uke tells the seme not to go, but the seme chases after the guy who pushed the uke, leaving the uke behind. The jerk! Yes, I know. Why in the world am I hooked onto this story? It's stuck in my mind somehow.

I've found it!

Yachin Hanbun no Ibasho desu, chapter 6, "Don't Be So Strict"

I have the same problem as you - too much manga - and would likely never have found it if I hadn't had a copy on my USB drive to read on my portable computer.

That's the one! Thanks a bunch!