I wouldn't say John's straightness is "likely" canon, as he said it multiple times and I think he has no reason to hide his sexuality

I agree, but if I say he is 100% straight I’ll get people going “He was only 13 when he said he wasn’t a homosexual, he could had just not figured it out yet, that’s young” or “well what about all that gay subtext he had with [insert your choice of Dave or Karkat here] back in the earlier asks” or even even something like “he can’t be straight that goes against my precious yaoi shipz >: (”