For like the first year and half of fandom, Sam was the bottom, and er...then he turned out to be more muscular than Dean. However, even during that first year, I thought Dean should bottom, for the exact reason that you stated, "WHY MUST THE MANLIER ONE BE THE TOP?" and honestly, Dean still is more stereotypically male.

But I really like the idea of Dom being a bottom, just because he takes EVERYTHING on himself, and he needs to learn to let go, and Brian could do that for him. But I'm a big fan of switching off, which I do understand is not really what happens in gay relationships, but nevetherless, I wants it.