To a certain extent I kind of agree with you, but on the other hand the idea that every character should be assumed heterosexual until proven unquestionably gay bothers me almost as much.
It bothers me too, actually. My gripe is about trying to make a platonic relationship more "meaningful" or "interesting" by involving sexuality at all, be it homo or hetero. I enjoy SM and WW's friendship, but I'd be bothered if a whole crowd of people conjectured that it's practically canon that they totes slept together when it's canonically established that Clark and Lois are in love and married. By the same token, I wouldn't like it if Dick and Donna became romantically involved in canon because I love their brother-sister dynamic.
Of course, I've been in fandom long enough that I no longer care what fans choose to interpret. It's no skin off my nose and all in good fun. But it took a long time for me and other fen to evolve that far. I still remember being persecuted by the "Draco Malfoy DOES NOT wear leather pants" Inquisition a couple of years ago. *snorfle*