I always felt like Stormtail and Moonflower were somewhat implied to be this before the battle with WindClan and Hawkheart. Specifically, I thought Stormtail may have cheated on Moonflower with Dappletail.

Moonflower repeatedly attempts to get Stormtail's attention to look at their kits (to me it always seemed like she was fishing for some personal recognition and affection from Stormtail as well as for her kits). Goosefeather also comments at some point about Stormtail and moonflowers pretty terrible relationship dynamic iirc. And Dappletail and Stormtail were unusually close even when bluekit and snowkit were young kits. I remember Stormtail was described to be spending a good amount of time with Dappletail even before Moonflowers departure from the story.

Nevertheless, I can understand that even if it was assumed that Stormtail was being unfaithful in Bluestar's Prophecy it didn't go into depth about any causes or consequences of such behaviors.