There’s a section of Spirou fandom, much in evidence on tumblr and DeviantArt, that is really into imagining various characters involved romantically. And one of the main pairings is, of course, Spirou/Fantasio.
There’s nothing particularly unique about that: you get the same kind of slash-fantasies for pretty much any piece of pop culture. But this particular case is perhaps more well-founded than most: Spirou and Fantasio, after all, are two guys of similar age who never (at least in Franquin’s album) show any interest in women, who spend most of their time with each other, and who eventually move in together. Even if it wasn’t intended that way (and it wasn’t), they could easily be seen as a couple.
However you choose to interpret their relationship – gay or straight, romantic or platonic – one thing is for sure: there is a love between them, and the strength of that bond is one of the main things that defines the series. Happy Valentine’s Day!