Well, yeah, people think that because that's most common. I've worked with the public a lot and its a natural reaction to assume a guy and girl together are 'together'. Its nothing of an 'unwritten rule', but it is in fact, most common. So since its more common that they are a 'couple' than that they are just friends, it makes sense to assume the same of others.
I've had repeated times when i was out with my friend and her children and people would tell us what great kids 'we' have. A few times i was told how much one or more of them looked like me and they weren't even my kids. We weren't married. Or people just assumed we were married, kids or no kids. At first we corrected people. Eventually we just left it, unless it was a situation where i may run into the people again. Its not worth being worried about or bothered by. I never even thought of it as uncomfortable. Just laughed and said 'nope, not us'.