I don't really know either. I do that in the wild, most "monoganous" animals cheat. Specificly, I know of whoel handful of bird species who mate for life, but still cheat, they just don't help raise the "other woman's" babies.
Honestly, I have never heard of fish mating for life in the wild and both parties involved choosing to be faithful. That doesn't of course mean it doesn't happen, just saying I have not heard if that personally.
Two roads diverged in a yellow wood, but thankfully they both led to fish stores.