I'm 99% sure that was a flashback. He was reminiscing about his time with that guy as they were saying goodbye. http://www.mangago.zone/read-manga/crystal_harem/mf/v02/c001/20/
So yay! No cheating. He couldn't really help it the second time, since he was abducted...
That's totally a flashback
Ah yes, I see it now. At first it looked suspicious because Ryou was hugging him and then it cut to a sex scene, but I missed the panel where the older guy was still sitting in his chair fully dressed as Ryou says goodbye. Goody.
That was actually a flashback. As Ryou was reminiscing about the "contract" they made, and they gave each other happiness during that time.
but too bad Ryou did a little cheating behind Yajima's back and slept with the old "contract" guy one last time (as a thank you, I think), especially since it was that guy who was killing the boys and wanted to kill Ryou as well. Thats the only part I didn't like.