Well, we've never heard Rodney or John specifically say, "I'm 100% straight, and I don't like to have sex with men." (Not that that would stop any of from writing slash, probably.) We *do* know, specifically, that they are running out of coffee and basic food sources, from statements made in canon.

I'm not saying that I think McKay/Sheppard (or any other SGA pairing other than Laura/Carson and John/Random Bimbo, for that matter) is canon, but it's a totally different issue.