I don't think the writers/network would make the relationship as close and as desperate as it is without blood relations being involved, if that makes sense? For fear of the big scary 'gay' label!

That's what I was thinking, really; if Sam and Dean weren't brothers but had the same level of intensity and devotion in their relationship, I don't think Supernatural would be perceived as anything other than a big gay love story. I'm not sure the relationship they have would even be possible if they weren't family.