I'm not going to say Masahiro is some purity angel, but...
A confession is not equal to a mutual agreement. Granted, Masahiro did once claim that Yoshimi was his lover, but that was more out of jealousy, and I think even Yoshimi understood that and didn't really take that as a pledge that they were dating.
But I digress. The point is, you're free to do as you want when you're not dating someone. You didn't make a promise to be loyal or faithful to them; you didn't establish your bond as something beyond platonic relationships; in short, you didn't make any commitments.
Hence, I don't really think it's fair to hold Masahiro responsible.
Also, keep in mind that a lot of their relationship /was/ forced by Yoshimi (he blackmailed him to instigate the physical relationship), even if Masahiro eventually came around to accept it. That makes it even more confusing for Masahiro to draw the line between what is fine and what is not.
Did no one notice that after somewhat confessing to Yoshimi he decides to allow Akito to try and fuck him "one last time"? Honestly all of the characters lacked purity, not just Yoshimo.