Would giving aid and comfort to the enemy constitute a crime that would affect one's inactive reserve status? Admittedly, Kerry was never tried with the crime of treason, but he was arrested (we have pictures) while participating in antiwar activities.
As a former naval officer, I just checked my records. I had a regular commission. Although I resigned my commission in November 1972 after almost 8 years on active duty, I didn't receive my Honorable Discharge until 1978, which just so happened to be on the same date as Kerry's February 16, 1978 honorable discharge date. It makes me think that there was some blanket discharge of personnel from the Standby Reserves.
Well, it quite possibly makes you unfit for duty. It is a crime under civil law. There might have been an issue that he was no longer fit for active call-up and so somehow had violated a reserve obligation. That would have been a bit of a stretch at the time.