No.Captain_Heavy said:is it wrong to assume that someone you meet is heterosexual?
Heterosexual people are in the vast, vast majority. Chances are by assuming they're heterosexual you're right. Furthermore, for the vast, vast majority of conversations you're likely to have, sexuality has no relevance (for example, if you're talking about toast, and whether or not you prefer raspberry jam or blackberry jam, sexuality has fuck all to do with that.)
If you want to be 100% on the sexuality of everyone you meet, you'll have to make it a question very early on in the conversation with them, and that's fine if you're planning in talking about things where ones sexuality is important.
Personally, I don't give a fuck what sexuality someone is - if they want to tell me about their sexuality then that's up to them, but in the meantime I'll assume they're *whatever they stereotypically look/act like*, but mostly I won't care - I don't generally look at someone and think "ah, he's straight/gay/whatever", because there's other things I prefer to care about (usually judging them on what T-shirt they're wearing).