The relationship between Darien (also known as Prince Endymion) and Serena (Sailor Moon) is a core element of the story, and their love for each other is central to the narrative. While they face challenges and misunderstandings throughout the series, Darien's loyalty to Serena remains unwavering.
It's important to note that there might be fan interpretations or theories surrounding this topic, but in the canon of "Sailor Moon," Darien does not cheat on Serena.