Keep It a Secret From Your Mother! – Chapter 100.5

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In the review chapter, the author talks about the relationships between Na-Yeon, Yeon-Ah and Soo-Yeon and the reasonings for lesbian sex in the series.

He shows off a diagram of his ideal harem relationship, which is for the girls to be involved with each other as well as the MC. This prompts Yeon-Ah to break the 4th wall, saying that Na-Yeon and Soo-Yeon need to have sex with each other.

So it’s just implied the girls have lesbian sex with each other whenever the MC isn’t around.