It starts because they wanted to get the attention of Rin and Sae, their crushes, and thought that hey why not be lovey dovey with other and see if that makes them jealous. It does, it makes the Itoshi brothers INCREDIBLY jealous especially since both RyuSagi are shameless in front of them in a desperate bid to catch their attention.

Before this whole schtick, RinSagi were on okay speaking terms while RyuSae were also on texting terms but that was it. RyuSagi also don't really know how to push the relationship further and confessing like normal people was below them so they didn't even try that route (the Itoshi brothers would iterate later on that they would've preferred that while RyuSagi would laugh in their face).

The thing was born because Isagi wondered what Rin was like jealous and Shidou jumped on that and went "hear me out" and it went downhill from there. They went on "dates" that was actually just them both talking about their respective crush and then talking about soccer. They play it up though when in front of Rin and Sae. Isagi lets Shidou kiss him on the cheek and vice versa but that's honestly the furthest they went for aside from hand holding.

The way they find out is like this. Rin finds out when he becomes an unwilling audience to seeing Shidou press the sloppiest kiss on Isagi's cheek while Isagi laughs. Rin was already halfway to screaming at Shidou while shoving him away from Isagi when Isagi literally presses a cute kiss back and his brain FREEZES. He just stands there, staring, unseeing because WHAT. Then, the jealousy hits and Rin is seething. He grabs Isagi and snarls at Shidou and drags Isagi away.

Sae finds out because he and Shidou are facetiming one time and mid call, Isagi pops up with food for Shidou. He grabs the back of Isagi's neck and they pretend to kiss but they're really just smirking at each other but well Sae is on a video call and he only really has one angle. He grips his phone tightly and then immediately books a flight for Japan because what. What is that. What.

There's a sprinkle of denial in the Itoshi brothers because of course there is but when it goes on for too long the jealousy is stronger. It gets to the point that Rin practically grabs Isagi and confronts him in his own room while Sae locks him and Shidou in a hotel together.

"Heh ehehe,"

"Don't laugh at me,"

"Not my fault you got sunburn trying to tan with me baby! Im more used to this shit than you are,"