There are many factors that can affect these triangulated relationships, and how they are combined can affect the outcome in different ways.
Time Elapsed A new separation is clearly more undefined.
The heartache that arises if and when those clandestine relationships are discovered never harbors a good outcome.
A partner who may have understood a one-night stand that is immediately confessed is less likely to feel as humiliated as one who finds out much later or when a relationship is more established.
Relationships that are new have not had the time for enough negatives to accrue that can outweigh the reasons to stay together.