Again, I have to just point to my comments above. It's not that it's the only explanation (which slash is almost never about, except for oh, say, Sentinel), but it can be read that way. I've seen shows with no slash subtext. This is not one of those shows.

I really don't think it has anything to do with people's knowledge of real sibling pairs. To say that all Wincesters are only into wincest through ignorance is a bit...much. Sort of like saying that guys that fantasize about doing it with twin sisters only have said fantasies because they don't know any real sisters.

As for why the show keeps calling them gay...I have no idea.
From "Something Wicked This Way Comes":
kid at motel: King or two Queens
Dean: Two Queens
kid: *smirks* I'll bet.