Hopefully cause it’s what irked me for the rest of the story any signs of cheating becomes unreadable for me
Same here =/
I cam to the conclusion that they slept together before the couple dated because no matter how good they are at hiding, the guy's concern about everyone dying is not impossible. With how obsessive the other guy is, I don't think it'll be easily possible to cheat - indeed, I don't think it's likely with constant surveillance and a very real threat on the people involved. I'm guessing he knows that the other guy wouldn't risk it. I already felt very iffy when I read the entire thing, that joke was still in rather bad taste, but I guess they're happy that way too. Made for each other, in a way.
It’s implied that it was before they got together. The band leader wouldn’t risk it.
But don’t take my word for it
I want to think that uke didn't cheat on seme for real, and that he had sex with that other band member before they (uke and seme) started officially dating... u.u