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Shihoon (aka "Sugar Daddy", aka "oppa") is gay. He is Mari's brother. Mari calls him "oppa".
Mari is seeing a married man. She calls the married-cheating man "My Oppa".
Junyoung hears "oppa" out of her mouth all the time and assumes "oppa" and "My Oppa" are the same person. They are not.
"My Oppa" (Mari's married-cheating boyfriend) went on vacation with his family and because of that, is not seeing Mari at the moment.
"Oppa" (Mari's brother) began seeing Junyoung at the same time that Mari is currently not seeing "My Oppa" (her married-cheating boyfriend) because he is gone with his family, so he thinks Shihoon (aka "Sugar Daddy", aka "oppa") is cheating on her with him.
Mari caught her brother (aka "Sugar Daddy", aka "oppa") kissing another guy a long time ago so she's knows he's gay. Shihoon (aka "Sugar Daddy", aka "oppa") does not know Mari caught him, so he doesn't know that she knows he's gay. SO-- he doesn't want Mari to know that he's gay, or about his relationship with Junyoung.
Junyoung thinks Shihoon (aka... ahh, you get it by now) doesn't want Mari to know about him because Shihoon is the married-cheating boyfriend (aka "My Oppa"...which he's not) and doesn't want Mari to think he's cheating on her.
tl;dr -- Mari calls her married-cheating boyfriend the same name as her brother, which is just confusing for everybody... except Mari.
Kay,, so ive read all the chapters in a jiffy. Can someone tell me whats going on? I thought the sugar daddy was married and was seeing mari, then the latest chapters he's mari's brother???