Responses
Not once has Kousuke ever mentioned being a romantic relationship with anyone other than Masahiro. He would have said something by now if that were the case. Meaning that yes, Masahiro is the only person that Kousuke has ever been in a relationship with.
Is Masahiro the only person kousuke has been with romantically? In chapter 26 (or around there) when masahiro was sad thinking kousuke had been with othe people and said i love you to them, kousuke said he had said i love you in different situations like friends and family - does that mean Masahiro is his first romantic love and his first time?