In case you missed it: on page 19, a second before she cums, Nyaruko calls for Mahiro-san. In that moment, Cthugha realizes that Nyaruko will never love her as much as she loves Nyaruko, or as much as Nyaruko loves Mahiro. So she decides to pull out on page 20. On page 21, Nyaruko demands an explanation. The following exchange takes place (very roughly, I only know very basic japanese)
Cthu: Nyaruko, you like that SHOUNEN? Nya: Wha? Don't change the sub-/Don't fuck wit- Cthu: Nyaruko, you really want to make babies with that SHOUNEN? Nya: Isn't it obvious? I love Mahiro-san, I want to have his children...! Cthu: ...I see, Nyaruko. Nya: ...? <rest of the page is too much for me. I will point out however that Cthugha mentions it was nice to be the first to have "Nyarubutt"> <p.22> Cthu: Someday I hope, Nyaruko... I'll have the privilege of being the one loved by you, instead of SHOUNEN. Cthu: When that time comes, I really want the two of us to have a baby, Nyaruko <3