"how would you describe fiction for it" to me means "what kind of label would be useful". Although there *are* people for whom canonical vs. non-canonical is the most important distinction, in this culture (in the US at least) the distinction between "willing to accept same-sex relationships" and "not willing to accept them" is far more emotional and important for most people.

Basically, I don't get the same "feel" from canonical same-sex relationships as I do from traditional slash, but since more readers sort by the "same-sex" label than by the "canonical" label, calling such stories "slash" is far more useful than calling them "gen".