No, I meant before reading Prunus Girl. What I meant by that was I never read shounen ai/yaoi so I didn't know yaoi/shounen ai manga play with the readers like that so I continued reading Prunus Girl

Actually, shounen ai/yaoi didn't play with the readers like that. They usually show it openly and most could be seen from the cover. (two boy "play" with each other or at least embracing/hugging).
Therefore, Prunus Girl Isn't a shounen ai/yaoi. It stays kinda neutral with just shounen and gender bender. at least until before the last/epilogue chapter which entitled "Prunus Boy"
Now thats what I called TRAP.