Most Helpful Guy
No. The only "reason" I can think of that people use is when they are in a bad relationship and reason with: "What does it hurt if the relationship is already sour?"
If your relationship is bad to where you no longer care for it, then end it. You have a serious mixup of priorities when you are running to someone else before even settling the first issue. End the relationship instead of stirring the pot and creating more drama.
Oh and I'll also add a counter-excuse to what people come back with when confronted with what I just said. Sometimes they'll say they aren't brave enough to confront their partner and end things. There are two things wrong with that:
1) You are brave enough to cheat on them. If you are afraid of them or just want to avoid awkwardness/conflict by ending things with them, then surely you should know that cheating on them will make them just as bad as just dealing with them yourself, right? Probably even worse than what they would be.
2) Hate to tell them but it's not going to be easy. Just because avoiding it due to awkwardness seems like the easier path, doesn't mean that it's the right path. It's going to take some actual effort to solve your problems. Cheating only adds to your list of problems.