How in God's name does a man wearing trunks equal "all-out boy scout" and standing up for all that is good?
Not a thing.
I assume the intended suggestion is that Supes wouldn't be concerned by looking silly because he's just a good guy who's not self-obsessed or trying to prove anything. Just doing the right thing is what matters.
None of which begins to explain why he'd choose the red trunks to begin with.
Or run around in tights and a red cape, for that matter.
Faora's outfit looks similar to Superman's other than the color. I guess the intention is to associate the way he dresses more directly with his Kryptonian background, another thing which the upcoming reboot is apparently going to do.