Technically this is a loophole
From Okarun's perspective, there's no point in "cheating" on Momo since they're not a couple yet. Momo, on the other hand, warned Okarun not to cheat on her, but she didn't say "I promise I won't cheat on you." It's very confusing and Yukinobu Tatsu decided to write it that way to create romantic tension between the two leads... Don't get me wrong, I'm sure Momo and Okarun will end up together. Though after this "warning," they're both bound to be faithful to each other and anyone who breaks the oath will be greatly hated.
From Okarun's perspective, there's no point in "cheating" on Momo since they're not a couple yet. Momo, on the other hand, warned Okarun not to cheat on her, but she didn't say "I promise I won't cheat on you." It's very confusing and Yukinobu Tatsu decided to write it that way to create romantic tension between the two leads... Don't get me wrong, I'm sure Momo and Okarun will end up together. Though after this "warning," they're both bound to be faithful to each other and anyone who breaks the oath will be greatly hated.