Uhm . . . because he, uhm, hugged Sirius. And . . . stuff. OBVIOUSLY Sirius/Remus is about as canon as a slash can go, at least before HBP. But if I wrote slash instead of reading it, I would totally make Remus's resistance to Tonks because he's still in love with Sirius a bit, and should be going out and loving men, or things. I do have to say that I never saw gay!Remus before fandom, but then I was young when I first came to fandom, and whereas now I go around saying "OMG GHEY!" every other page in some books, back then characters were much more "Straight unless the author says otherwise" to me. Because, um, well, it comes with this whole "being in high school" thing. And Remus/Sirius was one of the first goodslash I was exposed to. So.