Do most young men and teen boys dress like bums because they are afraid that people will think they're gay?

Most young guys wear baggy jeans and t-shirt which don't flatter their figures at all. Usually by the time they're in grad school they have a more refined look: on down shirts, hats, fitted pants etc. Is this because they're afraid people will think that they're gay if they dress nicely or just that they have no fashion sense