The macho posing is a bit ironical, and I think that's quite intentional on Bob's part. It is clear if you read between the lines that the young guy in the song is quite upset about losing his girlfriend...but he's pretending like crazy that he's not upset just to make himself feel better about it. And that's why the macho posing. It's outer defiance in the face of inner hurt.

The song was about Bob's on-again, off-again romance with Suze Rotolo, which he took very seriously indeed, and which mattered a lot to him. (And, yes, I know he was going out with Suze after the song was written and recorded...but it's still about that relationship. Like I said, it was on-again, off-again, and for quite some time. They separated more than once.)