It isn't 'wrong', but I don't know why it is necessary to assume anything unless there is a sexual moment (not between you and them, but just a moment in general). I mean, apart from their sexuality, there is no inherent difference between heteros, homos, demis, bis, as ... you know what, this 'shortening' thing works less and less the shorter the short-hand actually is.
But yeah, there isn't anything wrong with it, it is more like a 'safe bet', given that statistics are on your side (though context helps: if you are at a gay bar, the odds aren't in your favour), and there isn't anything wrong about making assumptions about someone's sexuality ... it is a bit redundant in most cases however. I mean, why bother assuming someone is straight if I want to ask them directions (or some other non-sexual course of events)?