You would appear to be correct very broadly, but, I played through as female V.

And the image shows one of the default presets for the female V.

You can't romance Panam as female V, not without mods, you have to be male.

You can romance Judy as female V, without mods.

Thus, arguably, Panam is canonically straight.

As best I can tell, you have to romance Panam for Johnny to say anything that might be construed as positive about Panam.

I say 'might be construed' because you have to assume that Johnny doesn't completely hate himself deep down, for maybe doing brash things very impulsively that led to horrific results... for comparing someone to himself to be considered positive.

Thus, Johnny's just being (or trying to be) nice to Panam because you as male V chose to like her.