The way I see things: Even when I don't know a person's gender, I tend to make an assumption, though occasionally my assumptions are wrong. This is because I see 'having gender' as a defining trait of just...being an individual human. We're not snails or jellyfishes, we're men and women. We're people! Thinking of someone as 'they' sounds...highly impersonal to me, like I'm thinking of a corporation and not a human being.