Responses
IDK I saw 2 cheating stories in your description.
So they may be not the one you're looking for.
Let's try anyway...
1 Both already have GF
http://www.mangago.zone/read-manga/bond_z/
2 They tattooed each other name
http://www.mangago.zone/read-manga/mukizu_ja_irarenee/
Oh thanks it was the second one
There's this one yaoi where the two main lead are in love with each other butbone already has a girlfriend.......by the end of the manga the one eith the girlfriend tattooed the the first letter of the other guys name and tge other dude did the same...... thats all I remember ifvyou know what it it please tell me （￣へ￣）