In my opinion, I think they only cheated with the older and younger version of their lovers. Notice that the OLDER RIN told YOUNGER SOUSUKE that when they were still students, he cheated on him with an older man (that implies the sex they are currently doing in the car)
And then the OLDER RIN told YOUNGER SOUSUKE that when they were adults, he cheated on him with a younger guy (that implies the sex between OLDER SOUSUKE and YOUNGER RIN)
I'm not sure about the 2nd one coz it's currently happening and I think OLDER RIN is pertaining to a past affair that SOUSUKE had.
In my opinion, I think they only cheated with the older and younger version of their lovers.
Notice that the OLDER RIN told YOUNGER SOUSUKE that when they were still students, he cheated on him with an older man (that implies the sex they are currently doing in the car)
And then the OLDER RIN told YOUNGER SOUSUKE that when they were adults, he cheated on him with a younger guy (that implies the sex between OLDER SOUSUKE and YOUNGER RIN)
I'm not sure about the 2nd one coz it's currently happening and I think OLDER RIN is pertaining to a past affair that SOUSUKE had.
Sorry for my bad english.