I've been thinking some more about this. I suppose I find it interesting that people could interpret the relationship between Sam and Dean this way, for partly the same reason I became a romance author, because I find the complexity of human emotion and relationships fascinating.
Why is it that men can't have an openly affection relationship without it being construed as sexual? For a time, the same conjecture was directed at Jack and Daniel on Stargate Sg1. It was said that there was a lot of gay sexual undercurrents to the fondness Jack and Daniel had for one another, in the same way Sam and Dean have been stuck with Wincest.
I think that Brokeback Mountain really blew a lot of the taboo surrounding gay relationships out of the water. And now authors such as Suzanne Brockmann are taking it a step further, by presenting gay relationships in mainstream fiction. However, obviously close, affectionate male relationships make a lot of people uncomfortable, makes them instantly assume that such an affection can only be sexual.
In writing the genre I do, I deal with affection in terms of sexual tension all of the time. However, the two are not unswervingly bound together. Intimate affection can be just that and nothing more. It's not always going to be wrapped up in lurid sexual lust. Despite affection and sexuality being closely related, in my opinion, they can and do often stand alone. In the same way that people can have sex, yet not have any affection for one another, people, (even men) can have affection for one another without there being any sexuality to it whatsoever.
Really, all I wanted to say was that I do find it very interesting that people can interpret a simple glance, an obvious brotherly love, a respectful admiration between men as gay.