One does not lead to another. Yes, there is correlation but no causation. There is correlation because people who are open-minded about one thing tend to be more open-minded about other things.
Even you have admitted that the probably most criticized aspect of the suit by non-fans is the trunks. People say they look like underwear, they look silly, feminine, etc. I personally don't agree with that. I like the trunks. I think they do a great job of breaking up the blue and giving the suit a sense of balance.
But sadly, we are not normal people. We are a bunch of pop culture/comic book nerds posting on a forum. We know that the trunks are inspired by the apparel of circus strongmen and are meant to convey strength and athleticism. Unfortunately, it can be argued that in mainstream consciousness the trunks no longer have those connotations. To many, they have the opposite visual.
Just because someone doesn't like the trunks, it does not follow that they do not care about the artistic intentions of Siegel and Shuster. Because of the changes in popular culture and fashion since 1938, trunks worn over tights no longer are associated intimately associated with athletic apparel. It is a perfectly reasonable opinion and LOYAL opinion to believe that for Superman's suit to have the visual effect on the audience intended by its creators, their design might need to be modified. The suit is supposed to make Superman look powerful, strong, brave, dashing, etc. That was Siegel and Shuster's intention and purpose in making it look the way it does.
Now, 70 years later, to many, the trunks give the opposite impression. Many fans are justified in arguing that removing the trunks better serve the visual intentions of Siegel and Shuster. It does not mean they don't understand the character. In fact, the arguments Kurosawa gives for keeping the trunks (like the appearance of masculinity) are often the same ones given for getting rid of them. This means that both sides get the character. They think Superman should look dashing, powerful, etc. People just disagree about how best to achieve that in 2011.