Souperman has really hit the nail on the head above.
The trunks derived from the original incarnation of Superman, drawn at a time when circus strongmen and musclemen wore trunks. Thus, that 'look' was seen as quite masculine, especially on a character who at the time was known primarily for his superstrength and stamina.
In modern culture though, it's come full circle. Trunks on top of leggings/trousers/etc look ridiculous and don't promote masculinity at all. If anything, they're the opposite ......... and imply a certain effeminacy and campness.
I get the argument that the trunks are part of Superman's traditional look, and some people are just totally opposed to changing that. But IMO, things like this - in films/comics/whatever - have to change, and have to move on. And they have to be relevant to modern culture and fashion. If they aren't, then they run the risk of alienating the audience and the item in question will either hold no interest for them, or become a running joke.
Superman as a character has been around for a long time now, and for all we know he could still be here in another 100 years. You can't honestly put your head in the sand and expect him to go forever with no costume updates or changes? We have to be open to change, provided that the change is a positive one and doesn't completely alter or ruin the fundamental aspects of the original concept.
If anything, we should be thankful that Snyder & co have managed to modernise Superman's look, add much more detail to the costume, and still have it looking instantly recognisable as Superman's outfit. They haven't bastardised it completely, unlike some of the concepts put forth whilst Warner were trying to knock together a Superman film over the 90s and early 00s.