It's not underwear.
Well, he wears it under his street clothes, so technically the whole thing is underwear. But yeah, the trunks themselves are trunks.
I assume Gunn kept the trunks because that's what Superman wears.![]()
Exactly. Why does he have a logo on his chest? Why does he have a cape? Why does he have red boots? Why does he have a belt on a one-piece bodysuit? It's because they're components of a Superman costume.
If anything, the cape is at least as anachronistic as the trunks. Capes and cloaks were standard garments for centuries, still often worn in the '30s and '40s as formal wear. So it seems inconsistent to complain about the trunks but not the cape.