I think that, until we do live in that kind of an ideal world, there's nothing wrong with making that distinction -- for clarity as well as other reasons. Unfortunately, people
do assume that couple/lover/anything = straight. Many gay people don't want that assumption to be made about them. Refusing to point it out can make a (political) point, and it's one I value, but at the same time, it often means less visibility.
Besides, it's a part of our identity; we take pride in it. And I just don't feel quite comfortable policing how people describe themselves and their own situations.
When it's done by straight people, I think it depends on context. In the context of wanting to legalize gay marriage, it's kind of hard to just call it marriage, since the distinction is part of the issue. In the context of something entirely unrelated, then yes, it can be weird to point it out, but... I think it depends.
I mean, I see your point and I don't outright disagree, but I think it's a little more nuanced than that.
Though none of that stopped me from totally adoring this quote:
http://bit.ly/gt31F3