
Spoilers for Drops 12-15 below
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No, they're not exes or former crushes. Kaburagi became friends with Sanjou in high school out of convenience (kept concerned teachers off his back, etc) and he feels guilty because he never considered Sanjou a real friend. Sanjou, for his part, thinks they were best friends and just seems to blush a lot in general (i.e., it's not connected to any romantic feelings). The fight Onoe and Kabu have is over Kabu sharing nothing from his past and Onoe's concern that he'll be ghosted like Sanjou.

Just a tiny bit more because (=' ,'=)
Additional spoilers for Drops 12-13
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Kaburagi acts so hinky when Sanjou is around Onoe because he sees the whole thing as a "minus point", a part of his dark past that he thinks will make Onoe think poorly of him. Hence why he keeps cutting into their conversations and is kinda freaked out when Onoe tells him Sanjou spoke with him. Sanjou is persistent because he's trying to recruit Kaburagi to work at his company and approaches Onoe to stir up trouble (thinking Kabu will reconsider working for him if things are uncomfortable with his coworker).
Kabu also confirms that Onoe is the only man he's ever dated and prior experience was for work.

you're the best!!!!! i don't want to think about kabu with ANYONE else one night stand/ for work sex so i've still been slightly traumatized by that but that's just my inexperienced self talking since most people that have been in different kinds of relationships would understand. that is so cute!!!! i love how the author has built their relationship. thank you SO MUCH for the translation <3!!!!
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wait guys, the last few raws, is that celeb guy kabu's ex???? or they at least had feelings for each other?? lots of flashback to their school days and blush faces.