I'm an underwear lover who has a 28 inch waist, so this topic very much pertains to me. To answer the question of whether I wear Boys XL or Mens S, it varies from brand to brand. Even though the two sizes are both listed as being the same waist measurements, it's not at all true (at least with the brands I wear).

When it comes to FTL, I wear Boys XL, because the Mens S is a tad too loose for me (again, even though they're listed as the same size, they really aren't).

Hanes is the opposite. Their Mens S fit me well, but their Boys XL are ridiculously loose. The two aren't even close to being the same size. I bought a pack of Hanes Boys XL once, and I swear they were so baggy it felt like I was wearing short boxers. Couldn't feel the elastic leg bands at all. I honestly believe that guys with a 36 inch waist could fit comfortably into them. In fact, when it comes to Hanes Boy sizes, medium fits me best, which is pretty absurd. Maybe the company feels that because childhood obesity is on the rise, they should make their briefs more loose to accomidate bigger butts. I dunno.

Jockey brand is the same deal as Hanes. The Mens S fit me well, but the Boys XL are too loose (although the difference between the two isn't as drastic as it is with Hanes).

BTW, fellow underwear lovers should check out my gallery. Lotsa pics of me wearing all different kinds of undies (briefs, bikini briefs, Pokemon briefs, panties, speedos...).

I love shameless plugs