SPOILERS BELOW
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It was a threesome. Lemon was the bottom.
Lemon liked black haired seme since they were in school. But the black haired seme only saw him as a competition.
The other dude, the pink haired seme figured out that Lemon liked the black haired seme and he just went along for the ride since the other two cannot communicate with each other without arguing.
Unfortunately since it only has a few chapters, it did not show the aftermath of the relationship between Lemon and the black haired dude when their vacation had been over and they had to go back to their normal lives.
But at least Lemon is now in his radar. Black haired seme is now starting to find Lemon attractive despite him not being his usual type.
Love Debugging