Kimi wa Pet: at the start, the main character has just broken up with a boyfriend of five years who was cheating and got some other girl pregnant. I guess you could consider pet-keeping cheating as well:

It's not just Sumire who has a pet, later on her love interest has one too and thinks he slept with her.

世界のどこかに必ず
キミの居場所がある筈

@ladykitty - thanks for the clarification! That's all I need to know lol - as long as it isn't the main couple.

Quote from tartufo

@ladykitty - thanks for the clarification! That's all I need to know lol - as long as it isn't the main couple.

it is the main couple but it was only a little cheating

what about skip beat? The guy doesn't outright cheat but he was just using her for his own self purposes and was dating other woman when the Kyoko(main girl) thought they were meant to be and the guy really was stringing them her along. [/spoiler]

    p.s "he"= shou   😛 

If you want a cheating male character, I highly suggest Piece of Cake by George Asakura, the same mangaka of Oboreru Knife. The story starts of slow, but it's amazing. And the art is okay at the start, but it gradually improves. Be warned, it has sexual scenes and the manga is still not completed. I was left hanging at the best bit! >:'(