I'd say "yes" on principle...
I say it's wrong because it's always wrong to assume anything about anyone, really. It's wrong to assume that all African-Americans are "Gangstas" in the same way that it's wrong to assume all girls like pink / ponies / pink ponies and that all men are bastards. OK sometimes assumptions can be justified but the circumstances have to be exceptional and by and large, assumptions are based on lame generalisations at best and often lead to unfair "pigeonholing" of individual people.
That said, society is slowly starting to realise that the lines between stereotypes are getting blurry. Several of my gay friends (I'm a fur - I'd say that over 75% of my mates are gay as a result!) are camp as you like but there are just as many who you'd never imagine as being gay in a million years if you were to meet them. This goes for both genders, by the way...
In short, while it's not nice to assume anything about anyone, nobody in their right mind is going to go to war on you for assuming they are straight until proved otherwise. It's a real stretch to call it ignorant or offensive unless one doesn't let it rest when they have their assumptions proved wrong.
As far as I know, every gay / lesbian / bi person I know has never really cared when / if people assume they are straight until told otherwise. It's how the person reacts after learning the truth that matters.
Wardy