Thanks to pointing out in the last chapter (or special) that there's an oddly high number of wetting incidents, which indicate a fetish on part of the author, I thought that Mahiro has an "accident" on page 5 at first.
This is from google so it could be wrong but this is what I found:
"In formal or polite contexts, “watashi” is gender neutral. However, when it's used in informal or casual contexts, it is usually perceived as feminine."
When I was studying Japanese in college, we were taught Watashi as the defacto pronoun to use. It can be used by anyone and no one will give you the sideeye.
@Eviltwin
Normally, Mahiro uses “Ore” when alone or with Mihari, but he uses “Watashi” around Momiji and the others to look more like a normal girl. It’s a notable detail in context.
@Extralie‘s google results are pretty much correct. A guy can use “Watashi” in a business context like during work, but it’s a bit weirder to use that in a casual context.
EDIT: Oh snap, I just realized that the line itself is just a reference to a movie called “I Want to be a Shellfish”, so the use of “Watashi” here is more likely just a joke rather than an actual slip or anything. That’s a big mistake on my part, sorry.