The Phantom, Batman and Wolverine wear their trunks (not underwear) in codtume. Why is Superman and ONLY Superman hit with silly* complaint?
*Silly: No credible basis.
Batman moved away from the trunks in the comics a while ago. Wolverine doesn't wear them any more either, and if he did, he's The ****ing Wolverine. He could wear a tut and it would be badass.
As for The Phantom, that's an argument?
I'm no fan of the trunks but capes aren't functional either, for any character, and "aesthetically pleasing" is a subjective thing. If this post is you just venting then fine but if you really want to attempt to persuade others in regards to the trunks you should make a better arguement.
Batman's cape is VERY much a functional tool. Not just in terms of gliding and cover, but in completing his intimidating image, which is one of the biggest tools in his belt.
Superman's cape is often justified, as it was in MOS, and similar to how much of his suit is justified, by the fashion and customs of Krypton. 'Hope' anyone?
Now, that said, the exact same could have been used to justify the trunks for Kal, especially in MOS.
First off, speaking in MOS specifically now, Kal's suit is centuries removed from "modern" Krypton, coming from the ancient scout ship. This is emphasized/showcased in the colours. Superman's suit has more colour than all other modern Kryptoinian's put together, lol.
Modern Kryptonians clothes are all but monochromatic.
Combine all that with the fact that, despite their claims that they tried many designs but could not make them work, we've seen some of those designs and they worked great. There's no reason modern Kryptonian's garments couldn't have included the seam lines around were their "trunks" would be, similar to some of the "trunkless" Batman designs, and even, to a lesser extent, the aforementioned Phantom, then Kal's suit could have simply had that area also a different colour. Same as how the background of everyone's crest on Krypton was the same colour as the glyph, but Superman's was a different colour from the S glyph itself.
Now, all THAT said, the MOS suit is probably the first trunkless Superman suit that I've seen that I think actually works, and looks good. I REALLY wish they would adopt it, or something more similar, in the books, over that atrocious 'Kryptonian battle armor' New 52 ****e.