Responses
It's implied.
MC is masquerading as a male student remember? The romance with MC and ML looks to be GAY to outsiders. Hence, "My fav Yaoi!~" being sarcastic.
Sarcasm is not all ways negative/rude, so when it's well known fact and yet one says the opposite, sarcasm is implied by tone. Tone is difficult to convy for some, much less in text.
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my fav yaoi