I guess I tend to think of slash, het and gen as mere descriptors. If there is same sex sex going on or an exploration of romantic feelings, I'd call a fic slash. If there is het sex going on or an exploration of romantic feelings, I'd call a fic het. If the fic has no sex and doesn't deal with romantic or sexual feelings, then I'd catetgorize it as gen.

I think I've actually labelled very few stories with sex or romance in them as gen; if I have it's because the sex is very brief and fleeting in an otherwise long fic and seems to have no impact on characterization. This happens sometimes for me when say Sam Carter picks up an OC in a bar. I would probably hesitate to label the same fic gen if the 7 word encounter was with Teal'c instead because that encounter HAS to inform Sam's characterization. Or the fic might mention canonical pairings but not focus on them or explore them. They are very backgrounded. I might label that fic gen.